The Craft of Economic Modeling_ Fourth Edition

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					         The Craft of
      Economic Modeling
                    Part 1
703




432




162

  1980   1985     1990   1995   2000   2005




                Clopper Almon



                FIFTH EDITION
                  January 2008
   The Craft of
Economic Modeling


           Part 1



       Clopper Almon

    Department of Economics
     University of Maryland
    College Park, MD 20742




         January 2008




         Fifth Edition
                         January 2008
Copyrighted by the Interindustry Economic Research Fund, Inc.
        P.O. Box 451, College Park, Maryland, 20740
                  Telephone 301-405-5609
                                                               CONTENTS


ACKNOWLEDGEMENTS..................................................................................................................iii

INTRODUCTION.................................................................................................................................v

CHAPTER 1. WHAT IS AN ECONOMIC MODEL AND WHY MAKE ONE?................................1
  1. An Accelerator-Multiplier Interaction Model of an Imaginary Economy.....................................2
  2. An Introduction to Identity-Centered Modeling ..........................................................................7
     Model 1: The Pure Accounting Model.........................................................................................9
     Model 2. A Behavioral Ratio for Personal Consumption...........................................................16
     Model 3. More Behavioral Ratios and Shortcutting Exogenous Variables................................20
     Model 4. Seeing Past Inflation...................................................................................................23
     Model 5. Modeling Behavior with Regression...........................................................................28

CHAPTER 2. BASIC CONCEPTS OF LEAST SQUARES REGRESSION.....................................37
  1. What is the Method of Least Squares and Why Use It?..............................................................37
  2. How to Calculate the Least Squares Regression Coefficients....................................................39
  3. Some Measures of How Well the Equation Fits..........................................................................42
  4. Measures of Autocorrelation and their Uses...............................................................................44
  5. Statistics for Each Independent Variable.....................................................................................46
  6. Matrix Notation for Regression..................................................................................................48
  7. A Shortcut to the Sum of Squared Residuals..............................................................................52
  8. Mexvals and Derivatives -- Measures of the Importance of Each Variable................................55
  9. Leverage, a Measure of the Influence of Individual Observations.............................................57

CHAPTER 3. INTRODUCTION TO G 7.3........................................................................................59
  1. Tutorial of the Basics..................................................................................................................59
  2. Digression on Graphs in G..........................................................................................................64
  3. Building a Model.........................................................................................................................66
  4. Using Help..................................................................................................................................70

CHAPTER 4. FORECASTS AND EXPERIMENTS..........................................................................73
  1. Counter-historical Simulation.....................................................................................................74
  2. Forecasting..................................................................................................................................77
  3. Debugging Models .....................................................................................................................81

CHAPTER 5. TECHNIQUES FOR IMPROVING A MODEL..........................................................85
  1. Working with the @cum Function..............................................................................................85
  2. Rates of Change with Continuous Compounding.......................................................................86
  3. Inflation I — The Wolf in Sheep's Clothing...............................................................................87
  4. Soft Constraints...........................................................................................................................90
  5. Inflation II — A Rough-hewn Timber........................................................................................92
  6. Inflation III — Accounting for External Shocks.........................................................................93

                                                                                                                                                  i
     7. Inflation IV: Using Distributed Lags..........................................................................................94
     8. Inflation V: Working in Levels instead of Rates of Change......................................................98
     9. Ways to Improve AMI..............................................................................................................100
         Improve the Investment Equations..........................................................................................100
         Improve the Savings Function.................................................................................................100
         Expand the Government Sector...............................................................................................101
         Improve the Foreign Sector.....................................................................................................101

CHAPTER 6. GOOD ADVICE AND BAD EXAMPLES...............................................................103
  1. Good Advice.............................................................................................................................103
  2. Bad Examples...........................................................................................................................105
  3. The Worst Example..................................................................................................................114

CHAPTER 7. THE THEORY OF LINEAR SYSTEMS..................................................................119
  1. The general solution of a system of linear difference equations..............................................119
  2. Frequency Response.................................................................................................................125
  3. Explaining the emergent cycle phenomenon............................................................................130

Index.................................................................................................................................................135




ii
                                    ACKNOWLEDGEMENTS

This book has been in the writing for twenty years or so, but the activity has been particularly lively
in the past ten years. The material has been used by a score or so of instructors and by several
thousand students. I am grateful to all of them for their patience with its shortcomings and
suggestions for improvements. The software which accompanies the book, G7, was written
primarily by the author but with many contributions and corrections by students and associates. In
particular, I mention with gratitude those who have helped in the development of the programs. This
group includes David Robison, Jane-Marie Mulvey, Pamela Megna, Douglas Meade, Kang Chen,
Jeffrey Werling, Qiang Ma, Qisheng Yu and Ronald Horst. Teaching associates John Sabelhaus,
Peter Taylor, William Stephens, Craig Hiemstra, Lesley Cameron, Lorraine Sullivan Monaco, and
Maureen Sevigny have made many useful suggestions. Douglas Meade and Daniel Marcin have
helped to port the newest version to Open Office 2.3.1 and update the text. Many, many others too
numerous to mention have made valuable suggestions.

Special thanks go to Wang Yinchu and a team at the Economic Information Agency of Jiangsu
province, who produced a Chinese translation of the second edition published by the University of
Nanjing Press. Similar thanks go to Paul Salmon of the University of Rennes who translated the
second edition into French and produced a French-speaking version of the programs. Likewise,
Alberto Ruiz-Moncayo and Luis Sangines have produced a Spanish-speaking version of the
programs while the first edition of the text was translated into Spanish under the direction of Javier
Lantero at Fundacion Tomillo in Madrid. Maurizio Grassini has been active in making the programs
known in Italy. His work led ISTAT to become the first central statistical office to use G data banks
for official dissemination of statistics, a purpose for which they are well suited. I am indebted to
Wladislaw Welfe of the University of Lodz in Poland and Maurizio Ciaschini of the University of
Urbino in Italy for the opportunity to conduct in their institutions week-long intensive seminars on
the use of G. Most recently, Yasuhiko Sasai has produced a Japanese translation with an application
to Japanese data. The 2003 and later editions benefit from a careful reading by Peter Taylor.

Finally, I am grateful to all my colleagues at INFORUM who have both encouraged the work on this
project and liberated me to pursue it.




                                                                                                     iii
iv
                                          INTRODUCTION

This is a book about how to build models of a business, an industry, or the whole economy. It
explains techniques used both in simple, single-equation models for forecasting the sales of a single
product of a single company and also in complex, many-equation models of an entire economy or of
the world. The principal example in the first two parts of the book is a macroeconomic model of the
USA. The title, The Craft of Economic Modeling, emphasizes that the book does not stop with the
theory or even with a few examples. Rather, it leads the reader directly into practice, for it is
accompanied by computer software and data that enable the user to apply to practical problems
every technique described. Though some of the exercises are just drills in the techniques, many call
for originality and creativity in handling real-life problems. Of course, as in any craft, a book can
only help, not replace, a living teacher.

The computer software, the G regression package, version 7.3, (referred to as G7) and the Build
model builder, are comprehensive, easy-to-use programs that run under Windows 95, 98, 2000, XP
and NT. They are designed for work with time-series data. Public domain versions accompany the
book or are available via Internet (www.inforum.umd.edu), where thousands of economic time series
are also available as data banks for G. Assembling equations into a model requires the use of the
Borland C++ compiler, which is also available for free download from
     http://www.codegear.com/downloads/free/cppbuilder.
All software and data are also available on a CD-ROM from the author at nominal cost.

Simply put, an economic model is a set of equations which describe how the economy or some part
of it functions. In my view, a model should incorporate and test our understanding of how the
economy works. Its equations should make sense. And it should be possible to test how adequate
our understanding is by running it over the past and seeing how well it can reproduce history. By
changing some of its assumptions and rerunning history with the changed assumptions, it is possible
to analyze the effects of policies. Finally, it should be useful not only for policy analysis but also for
forecasting. By studying the errors of the forecast, the builder of the model may hope to improve his
or her understanding of the economy.

I must warn the reader that these simple views are by no means shared by all -- or even by most --
practitioners of model building. The vector-autoregression (VAR) school gives little or no weight to
the equations expressing any sort of understanding of the economy; ability to reproduce the past,
however, is of great importance. The computable general equilibrium (CGE) school gives great
weight to the equations making sense but has little interest in testing the dynamic properties (if any)
of its models or in the equations fitting more than one point. In my view, each of these schools is
right in what it values but remiss in what it neglects.

Some of the equations in our models have constants which describe the behavior of firms,
consumers, or other economic agents. These constants, often called "parameters", must somehow be
estimated. The most frequently used way is by choosing them so that the equation describes
accurately the behavior of those agents in the past. Thus, estimating the parameters is just a way to
sum up the lessons of the past to forecast the future or examine how the past itself might have been
different had different decisions been made. A large part of this book is about how to do that

                                                                                                         v
summing up. But it is just as important to choose judiciously the variables to be included in the
model. Unfortunately, there is little theory about how this choice should be made. I hope that the
discussion in this book will at least help to make the choice a conscious one.

Model building is a serious business because models are playthings. If that sounds paradoxical,
consider that we can, fundamentally, learn in three ways: from our own real-life experiences, by
hearing or reading about the experiences of others, and by play. Indeed, zoologists have observed
that one of the distinguishing characteristics of Man is the adult's retention of the ability to play.
Therefore, in Man, even adults can learn. In economic policy, no one has enough personal
experience to be of much value; and evaluating the experiences of others is difficult because many
influences were at work in producing the actual historical outcome. That leaves play. But in the
past it has been impossible to "play economy" without playing with the real economy. Models and
micro computers, the super playthings of adults, open up the possibility of learning about how the
economy works through play. Personally, I readily admit that nearly everything I think I know
about how the economy works I have learned through play with models. When I read books or
papers about the economy written by people who have not played in this way, I am often struck by
how one-sided their knowledge is. Yes, I think that play with models is the best way to raise the
level of public understanding of how the economy works, and that the quality of the economic
policy we actually adopt depends on that public understanding. The attempts to develop that
understanding by exposing college students to clearly written texts and objective multiple-choice
examinations have failed. Maybe play can succeed.

When computers first made economic modeling feasible, the mystique of the machine raised
expectations that models built with it would prove as infallible as the machine's arithmetic. In fact,
the record of forecasters using models looks very human. That is, of course, exactly what one
should expect. But expectations are seldom rational, and when it became clear that models were not
the panacea that would cure all the ills of a business, some disillusion set in. Many bad models have
been built, some of them by large commercial firms in the modeling business. Sometimes the
salesman got the better of the scientist in the claims their builders made for them. However, I
believe that those who know how sensitive a model can be to the judgmental decisions made in
building it have not been disappointed in models, for they have not expected what could not be
delivered. Working through this book will offer the first-hand experience with models that enables
one to judge what can and what cannot be expected of them.

I should perhaps advise you that a large fraction of the economics profession regards such modeling
as we will do here as passé. This widespread opinion is based on a misreading – or non-reading –
of a well-known article by Robert Lucas which seemed to say that models such as those we will be
building “can, in principle, provide no useful information as to the actual consequences of
alternative economic policies.”1 In fact, Lucas had in mind a particular group of models in use at
that time (1975), and he actually went on to show how they could be modified to give what he
considered correct answers to such policy questions. His real point was not that models are useless
for policy analysis but simply that the fact that a particular model has a good “track record” in

1 Robert E. Lucas, “Econometric Policy Evaluation: A Critique” in The Phillips Curve and Labor Markets, Karl
  Brunner and Allan H. Meltzer, editors, Amsterdam, North Holland, 1975.

vi
forecasting does not mean that it will give correct answers to questions involving policy changes
that invalidate some of its equations. That point is surely accepted by all model builders and users,
both then and now. It by no means invalidates the use of models; in fact, quite to the contrary, it
shows that models are necessary so that the required changes in policy can be explicitly made.

Any policy analysis requires a model, for it requires that we compare how the world would be with
and without the change of policy. The only question is whether the model by which we make that
comparison is explicit, subject to scrutiny and testing, or implicit, vague, held somewhere in our
head and subject to change as our mood changes. Historical facts never replace a model for policy
analysis, for history gives us results only with or without the policy, never both with nothing else
changed.

For myself, I can hardly imagine how any systematic progress can be made in economic
understanding, forecasting or policy analysis without use of quantitative models. I hope that this
book can contribute to lifting the level of applied modeling and to a recognition of the judgment and
skill needed to build and use them well.

Over forty years of experience as an applied model builder have affected my choice of material for
this book and its presentation. As it is usually taught, econometrics is a series of exercises in applied
probability theory. I made a significant investment in learning this material in the expectation that it
would prove useful in building meaningful models. That expectation has been largely but not quite
totally disappointed. Nor am I alone in this experience; most applied model builders with whom I
have dared to discuss the matter have heartily agreed. Such a statement is a plain declaration that
"the emperor has no clothes" and is not likely to persuade teachers of econometrics to adopt this
book as a text, unless, of course, they want the students to know the truth.

This book emphasizes the simple, direct techniques and the common-sense analysis that I have come
to trust. These and only these are presented in Part 1 of the text. As a teacher, I have found that it is
fairly trivial to get a student to learn and apply some "sophisticated" technique; it is a major
accomplishment to get him or her to think about whether the equation and the estimated parameters
make sense. I have tried to coach the reader in such thinking.

The principal value of the probabilistic theory has been to show how, in certain situations, the
methods of Part 1 may lead to deceptive results. Understanding those possibilities is important, so in
Part 2 we look at the probabilistic theory and present some of the widely-used techniques, such as
generalized least squares and two-stage least squares, which are based on the probabilistic
assumptions. Through applications of these techniques to actual problems with actual data, the
student can form his own opinion of their value.

Part 3 turns to multisectoral models, models that distinguish different industries, such as chemicals
or textiles. All of the techniques developed in Parts I and II remain useful, but must be supplemented
by ways of handling the relations among industries.




                                                                                                      vii
A Guide to the Book

Before plunging into a theoretical discussion of any subject, one should begin with acute and
sensitive observation of the phenomena it concerns. In the case of macroeconomic modeling, that
means looking at the time series of economic data such as the National Income and Product
Accounts (NIPA), employment, interest rates, money supply, exchange rates, and prices. The
"nipashow" file, which comes with the software for this book, provides a good opportunity to
practice such observations. Once the G7 program is running, it is only necessary to say "add
nipashow" to begin this display of economic data. At first, nipashow asks questions about the
graphs it shows, then it simply shows other graphs and asks what the viewer sees interesting in
them. Finally, it leaves both the drawing and the asking of questions up to the user. After the
experience of viewing these graphs, one may well ask, How can the movement of these variables be
explained? And that is where the book proper begins.

Chapter 1 illustrates the idea of a model and shows how a simple model of the economy can
generate business cycles. It first uses a hypothetical model for which the reader can easily compute
the solution. It then introduces a series of five models of increasing usefulness. This series
illustrates identity-centered modeling, modeling that puts definitions and simple “behavioral ratios”
in center stage. Model 1 incorporates only the definitional equations of the NIPA and affords a
good review of the concepts in these accounts. Models 2 through 4 demonstrate the use of
behavioral ratios to make the models richer and more useful. Model 5 introduces a few equations
with parameters estimated with regression analysis from data on the American economy. Model 5
illustrates a fundamental point: it is not enough to have explanations of all of the parts of an
economy to have an explanation of the whole. The dynamic interaction of individually sensible
parts can generate quite surprising -- indeed, totally nonsensical -- results. Building a good model is
far more than putting together equations which individually satisfactorily explain each component
of the economy. The first section of chapter 1 does not require the use of computers. It can be
discussed in class during the same week in which the "nipashow" plots are being studied.

Chapter 2 explains least-squares computations and the display of results shown by G7. Chapter 3
concerns how to use G7 and particularly, how to estimate the parameters of model equations by
regression. Chapter 4 shows how to combine the equations into a model and has you do some
experiments and a forecast. It also explains how to use the numerous diagnostic techniques made
available by the software for spotting errors in your model. The experiments reveal some
weaknesses of the model, and Chapter 5 discusses ways to eliminate them. It closes with a number
of suggestions on how you can use the techniques now at your disposal to make significant
improvements to the model of Chapter 1. Chapter 6 gives some simple-minded maxims for getting
reliable, trustworthy regression equations. Most of these maxims are too simple to be mentioned in
a self-respecting textbook of econometrics. But they are the result of commenting on the work of
hundreds of students over the years. I found myself saying the same thing over and over until I
finally put pride aside and wrote down what I had been saying. Probably you will say to yourself as
you read it, "That is obvious; I don't need to be told that." Fine, but it is terribly easy to let these
errors slip in. Some very distinguished members of the economics profession have done so. I urge
you to measure every equation of your model against these simple criteria.


viii
Chapter 7 gives explains the basic mathematical theory of linear models. Its role is to help
understand some of the properties that the student may find appearing in his models, particularly
their tendency to generate cycles.

Part 2 begins with a fairly comprehensive quarterly aggregate model of the United States. In
building it, I have, I believe, learned a lot about how the economy works. I have even come to the
slightly arrogant opinion that no one should trust his understanding of how the economy works who
has not put that understanding to the test of building a model. In this model, I have endeavored to
get at structure and avoid reliance on lagged values of dependent variables. Thus, the model is
called the Quarterly Econometric Structural model, or QUEST. Actually, QUEST connotes more
than it abbreviates. Because all of the files for estimating and building QUEST are given, you may
use it as the starting point for your own quest to build a model of the economy. Many people have
felt that they learned to understand the human face or figure by modeling it. I have found the same
to be true of modeling the economy. I hope that in working with QUEST you will also feel that you
have joined in the search for a better understanding of the economy. QUEST also provides good
examples of equations to work with in the rest of Part 2.

Part 2 continues with an explanation of optimization in models in Chapter 9. This optimization may
be used either to improve the fit of the model in the historical period or to design optimal economic
policy. The important technique of nonlinear regression is explained and illustrated in Chapter 10.
Chapter 11 outlines some of the relations between probability theory and least-squares regression.
This relationship of rather modest importance to the applied model builder has been magnified by
the academic establishment to formidable proportions. The Datamaker fable, by which this subject
is here introduced, is one of the best-loved sections of the book by many readers to whom this
material has been presented with a straight face as if it ought to be believed. Basically, this fable
says that if we are in an ideal case, the least squares we have used in Part I is, in fact, the best way to
estimate equations. The rest of the chapter offers some methods, based on probability ideas, that can
be tried when the ideal conditions are clearly not met. These techniques include the Hildreth-Lu
procedure for dealing with auto-correlated residuals and "seemingly unrelated" regression and
stacked regression with constraints across equations. Methods of estimating equations with moving-
average errors are explained. The old subject of simultaneous equation bias and the new subjects
vector autoregression, cointegration, and unit roots get just enough attention to show how they relate
to the modeling approach used here.

Chapter 12 explains how to run a model many times with artificially introduced random errors. It
helps to answer the question of how much confidence we can have in a particular model.

Part 3 extends our scope to multisectoral models. It first explains basic input-output analysis in the
ideal case and then some of the practical problems arising in actual application. Special techniques
for modeling consumer behavior, investment, and imports are then considered. The Interdyme
software for building multisectoral models is then explained and illustrated.




                                                                                                         ix
x
         CHAPTER 1. WHAT IS AN ECONOMIC MODEL AND WHY MAKE ONE?

The tax cuts and high defense spending of the Bush administration, combined with relatively slow
economic growth led to growing federal deficits from 2001 to 2004. Over the same period, US
exports shrank and imports ballooned, spelling hard times for many manufacturing industries. Was
there a connection between these two developments? Specifically, did the tax cuts cause
manufacturing's problems? Or were they caused by a loss of competitive ability or drive?

The argument that the federal deficit was responsible for the difficulties of industry is simply that the
deficit led to high real interest rates, which led to a rise in the dollar, which made American goods
expensive abroad and foreign goods cheap here, and consequently stifled exports and promoted
imports.

Such reasoning points to theoretical possibilities. Whether or not these possibilities have been
historically important is another question, one that theory cannot decide. To answer it, one must
quantify, in the first case,

     the effect of the federal deficit on interest rates,

     the effect of interest rates on exchange rates,

     the effect of exchange rates on exports and imports,

     the effect of interest rates on investment, personal income, and saving.

Moreover, though the tax cuts may have had a negative effect on industry through these effects, they
increased after-tax income of households, who undoubtedly spent it in part on products of industry.
When all of these effects are quantified and combined into a system of equations, one has a model.

That model -- that is, those equations -- can then be solved under alternative assumptions about the
tax rates. First, one may use the actual tax rates and solve the model. The result is called a historical
simulation. It will not give exactly the historical values of the variables in the model, for the
equations which describe the behavior of people will not be exact. However, if the historical
simulation is not a pretty good approximation of reality, we had better take our model back to the
drawing board before going further. Assuming that it passes the historical simulation test, we then
change assumptions about the tax rates -- maybe this time we leave them unchanged from the 1990s
levels -- and again solve the model. This time we get what might be called a counter-historical
simulation. Comparison of the historical simulation and the counter-historical simulation will
answer the question; “Did the tax cuts stifle manufacturing?”

In economics, it is, in general, impossible to answer such questions of cause and effect without some
kind of model. The natural sciences allow for controlled experiment, a luxury we economists can
seldom afford. Building and running models under various assumptions is our version of the
controlled experiment. It must be emphasized that history alone can never answer a question of
cause and effect, for history gives us only one "run" of the economy. To speak of cause and effect in

                                                                                                       1
economics, one must have in mind some kind of model. It may be an entirely subconscious model,
or a model spelled out in theory but never quantified or tested, or an explicit, quantified model that
has been subjected to careful testing and scrutiny. I need hardly add in which sort I have the greater
confidence.

In Part II, we will build a fairly comprehensive aggregate model of the US economy and will apply it
to such questions as we have been discussing. But the model will also have much wider uses. In
particular, it will be useful for forecasting and for analyzing the effects of policies in advance. In
1993, for example, we could have studied whether the controversial Clinton budget proposal was
likely to stimulate the economy or choke off recovery just as it was getting started. We could ask:
“Given expected future real federal spending, what level of taxation is necessary to balance the
federal budget on average over the next five years?” We can examine the effects of tight money or
easy money; we can see what happens if productivity is retarded or accelerated; we can look at the
effects of stronger or weaker exports. We can ask what effects the stock market has on the economy,
and vice versa. In short, we can examine most of the major questions of macroeconomics.

If you, like me, find these questions both fascinating and important, then come along on the path that
leads to a way to answer them.


    1. An Accelerator-Multiplier Interaction Model of an Imaginary Economy

To illustrate what is meant by a model, let us begin with one so simple that we can easily calculate it
by hand. To keep the numbers simple, we will just make up the constants in the equations, textbook
style, out of thin air with a cursory glance at the American economy in the 1990's. To describe even
this simple model, it will be necessary to use two rather long words of Greek origin. Namely, we
must distinguish between the exogenous variables and the endogenous variables. The exogenous
variables are used by the model but are not determined by it; the endogenous variables are both used
in the model and are determined by it. (The Greek root gen means "birth" or "born". The exogenous
variables are "born" outside the model; the endogenous, within it.) Population and government
spending are exogenous variables in many models; income and investment are typical endogenous
variables in macroeconomic models. A system of equations which relates this quarter's values of the
endogenous variables to the current values of the exogenous variables and previous values of the
endogenous variables is, in economic terms, a model as we shall use that word. In more precise
mathematical terms, it is a system of difference equations. By calculating the simple model we
present below with a pocket calculator for a few quarters, you should get a good idea of how a
system of difference equations can generate a time path of the endogenous variables that looks at
least reminiscent of the actual course of the economy. You will also discover the remarkable fact
that the endogenous variables may show a pronounced cyclical behavior even though there is no
trace of a cycle in the exogenous variables.

Consumption in this simple economy can be described by the equation

(1) C = .6*Y[1] + .35*Y[2]


2
where C is consumption, Y is disposable income, and Y[1] is disposable income one period earlier,
read as "Y lagged once."

Fixed investment follows the “accelerator” theory of investment which makes investment for
expansion depend on past growth in peak output. Thus, to continue our simple economy,

(2) I = R + 1.0*ΔPQ[1] + 1.0*ΔPQ[2] + .1ΔQ[1],

where I is gross investment, Δ indicates a first difference (ΔQ = Q - Q[1]), R is replacement
investment, and PQ is the current or previous peak output, thus,

(3) PQ =       Q         if Q > PQ[1] or
    PQ =       PQ[1]          otherwise.

The first term of equation 2 represents, as already said, replacement investment. The next two terms
represent expansion of fixed capital when output rises above its previous peak, the output level for
which we already have adequate capital. The final term in equation 2 represents inventory
investment. Expansion investment -- from the way peak output is defined -- cannot be negative;
inventory investment, on the other hand, can be either positive or negative.

Imports, M, are given by

(4) M = -380 + .2(C + I + X)

where X is exports, which are exogenous.

The output variable, Q, is

(5) Q = C + I + G + X - M

where G is government spending. Finally, disposable income is roughly Q less taxes, which we take
as 28 percent of Q, so

(6) Y = .72*Q.

The exogenous variables in this simple model are:
       G     government spending
       X     exports
       R     replacement investment.

All others are endogenous. If in some period the exogenous variables and the required lagged values
of the variables Y, PQ, and Q are known, then equations (1) - (6) can be applied, one after the other
in the order given, to get the values of all the endogenous variables for that period. Then one can go
on to the next period, and the next, as long as the exogenous variables are known. Thus, given
starting values and exogenous variables, the equations can be used to calculate the course of the

                                                                                                     3
economy over time. This simple fact is so fundamental that it is a good idea to impress it upon
yourself by seeing it happen in your hands, rather than inside a somewhat mysterious computer.

Table 1.1 therefore provides a work-sheet for calculation of this simple model. At this point, you
should get out your pocket calculator and compute at least a few lines. Since you need three lagged
values of PQ to get started, the first line that you can calculate is line 4. All values, however, are
shown for lines 4, 5 and 6, so you can check that you have understood the formulas. (Do not worry
if your calculated values are different after the decimal from those shown in the table. The table was
calculated by a computer carrying more decimal places than shown here, and the numbers were then
truncated in printing.) By line 6, this economy is in recession, as you can see from the Q column. In
line 7, some values are missing and need to be computed by you, but the total output, Q, is left as a
check. By line 8, you are entirely on your own except for the exogenous variables. Their values, by
the way, are straight-line approximations of their values from 1979 to 1992 in the American
economy. Places for you to put calculated values are marked by a decimal point.

I urge you as strongly as possible to calculate a few periods of Table 1.1. until the next peak is
reached and the economy turns down. Endeavor to explain why the economy turned up and why it
turned down. If you understand how this simple model works, you will have no great difficulty
with the more complicated models which follow. If you miss the point here, trouble lies ahead.

The economy is in recession when you begin. As you calculate, you will see it grow modestly until
it passes its previous peak. Then the accelerator terms in the investment equation will drive the
economy up steeply. Then growth slows; but as soon as it slows down, investment drops, and slow
growth is turned into decline. This model will thus show a significant cycle. Figure 1.1 shows the
course of Q which it generates. Notice, however, that there is no cycle in any of the exogenous
variables. They all grow along perfectly straight lines, as you can see by noting that they rise by the
same amount each period.

We have thus discovered the important fact mentioned above: the equations of economic behavior
can generate cycles where there are none in the exogenous variables. From Figure 1.1, you will have
noticed that the cycle is persistent and of about a constant amplitude. This is a characteristic of a
non-linear model. Equations (1) - (2) and (4) - (6) are linear, that is, the dependent variable on the
left is just a sum of terms each of which is just one of the other variables possibly multiplied by a
constant. Equation (3) for peak output, PQ, is, however, a horse of another color and not at all linear.
If all equations of a model are linear, the model itself is said to be linear, but so much as one non-
linear equation and the model becomes nonlinear. This distinction is important because it may be
shown mathematically that in a linear model any cycle will, in all probability, either fade away or
grow explosively. Only a most particular constellation of values of the constants in the equations
can give the sort of steady, persistent cycling behavior shown in Figure 1.1. Thus, in the
construction of models, we should pay particular attention to the non-linearities.




4
                   5220
                                     Fig 1.1: Q in Simple M odel




                   4445




                   3670


                                   1980                    1985               1990



You should not conclude from this example that all such examples will necessarily show this sort of
cyclical behavior. In models with strong accelerator investment equations, however, cycles are not
unlikely.

                                                Exercise

1.1.( a) Calculate the course of the imaginary economy until Q turns down again. ( b) Explain why
      this economy turns down when it has been growing, and why it begins to grow after a few
      periods of decline. Remember that everything there is to know about this economy is contained
      in the equations. Do not bring into your explanation factors like interest rates or inflation which
      play no role in this economy.




                                                                                                       5
    Table 1.1: Worksheet for Hand-calculated Examples
    Line   X     G       C         R       I       M       Q         PQ       Y


     1   303.9 714.9   2653.6    518.7   631.9   337.9   3966.6    3966.6   2855.9

     2   308.8 718.5   2696.7    522.5   653.6   351.8   4026.0    4026.0   2898.7

     3   313.8 722.2   2738.8    526.2   648.2   361.8   4061.2    4061.2   2924.0

     4   318.7 725.9   2769.0    530.0   628.1   363.1   4078.6    4078.6   2936.6

     5   323.6 729.5   2785.3    533.7   588.0   359.4   4067.2    4078.6   2928.4

     6   328.5 733.2   2784.8    537.5   553.7   353.4   4047.0    4078.6   2913.8

     7   333.5 736.9         .   541.2      .       .    4033.7    4078.6   2904.2

     8   338.4 740.5         .   545.0      .       .          .       .          .

     9   343.3 744.2         .   548.7      .       .          .       .          .

    10   348.3 747.8         .   552.5      .       .          .       .          .

    11   353.2 751.5         .   556.2      .       .          .       .          .

    12   358.1 755.2         .   560.0      .       .          .       .          .

    13   363.0 758.8         .   563.7      .       .          .       .          .

    14   368.0 762.5         .   567.5      .       .          .       .          .

    15   372.9 766.1         .   571.2      .       .          .       .          .

    16   377.8 769.8         .   575.0      .       .          .       .          .

    17   382.8 773.5         .   578.7      .       .          .       .          .

    18   387.7 777.1         .   582.5      .       .          .       .          .

    19   392.6 780.8         .   586.2      .       .          .       .          .

    20   397.6 784.5         .   590.0      .       .          .       .          .

    21   402.5 788.1         .   593.7      .       .          .       .          .

    22   407.4 791.8         .   597.5      .       .          .       .          .

    23   412.3 795.4         .   601.2      .       .          .       .          .

    24   417.3 799.1         .   605.0      .       .          .       .          .




6
    2. An Introduction to Identity-Centered Modeling

As we move from a simple imaginary economy to a real economy, we immediately encounter two
complications:

     The accounting system becomes much more complicated with many more variables and
         identities relating them.

     The behavioral relations are no longer simply made up but must be induced from an interaction
         of our imagination with historical data provided by the national accounts and other
         sources.

In the rest of this chapter, we will go through a sequence of five models of increasing usefulness,
each built from the previous one.

The first four models are very similar to those that have found wide use in business in the form of
spreadsheet calculations such as those performed by Excel, Lotus and similar programs. Today,
nearly anyone planning a conference, a school budget, or a complex business deal wants to see a
spreadsheet to judge how the finances will come out. These spreadsheets are what may well be
called tautological models. Given their assumptions, they are sure to be correct. They express their
assumptions usually as simple ratios and employ simple addition or subtraction to total up the
revenues and the costs and arrive at the bottom line with utmost clarity. Our first four models are
patterned on these spreadsheet models, and their great virtue is their simplicity and ease of
understanding. They stress, however, the part of model which is most certainly correct, the
accounting identities. For this reason, I speak of this approach as identity-centered modeling.

The fifth model goes beyond what it is easy to do in the spreadsheets, namely, it introduces
behavioral equations estimated by linear regression, that is, by fitting a mathematical equation to
historical data. Relations found in this way can express much more complicated economic behavior
than can the simple ratios used in previous models. Through them we can hope to find, for example,
the way that investment is related to numerous factors such as growth in output, wear out of
capacity, and costs of labor and capital. Models that use these equations can give a much richer
description of economic behavior than can the tautological models. But the behavioral equations
will not fit perfectly, so -- unlike the tautological models -- these models do not perfectly reproduce
the past. How well they can reproduce it, therefore, can become a criterion in judging them.

Model 1 just sets out the variables and identities that are central to the system of national accounts
used in the United States. We may call it a pure-accounting model because it makes connections
among variables based only on accounting definitions, not on economic behavior. It is just a
skeleton and would give strange results if we tried to make it walk. Moreover, it has many, many
exogenous variables so that it would be awkward to use in practice.

Model 2 introduces a single, obvious but centrally important behavioral relation into the model,
namely, a relation between Personal disposable income and Personal consumption expenditure. The
relation which we introduce is extremely simple, namely a ratio, fixed in any one period but variable

                                                                                                         7
over time, between consumption and income. Because such a ratio expresses economic behavior, it
is natural to call it a behavioral ratio. This ratio then becomes an exogenous variable while the
quantity in its numerator becomes an endogenous variable. The introduction of a behavioral ratio
does not reduce the number of exogenous variables. Rather it replaces one exogenous variable by
another which is likely to be more constant, perhaps without a trend, and easier to forecast. For
example, if you ask me what Personal consumption expenditure will be in 2050, off hand I would
have no idea. But if you ask me what the ratio of Personal consumption expenditure to Personal
disposable income will be, I could answer with some assurance, “Oh, about what it is now,
somewhere between .9 and 1.0.” Model 2 will also allow us to make a meaningful multiplier
calculation, that is, to answer a question like, If government spending goes up by 10 billion, how
much will GDP go up? The Model 1 answer would be 10 billion; Model 2 has something more
interesting to say.

Model 3 carries further the process of exogenous variable replacement by means of behavioral
ratios. It also shows how certain exogenous variables can be lumped together to reduce the total
number.

Model 4 introduces another useful method of exogenous variable replacement, namely to make the
variable exogenous in constant prices. Models 1, 2, and 3 worked only in current prices; model 4
introduces deflators so that we can also talk about both the rate of inflation and variables measured
in real, constant-price terms. Forecasting of exogenous variables is often easier in real rather than in
nominal (current-price) terms. For example, Social Security benefits can be calculated in real terms
from the age structure of the population and the payment schedule. The dollar value of the
payments, however, will depend on the increases in the Consumer Price Index (CPI) between now
and the time they are actually paid. That increase is something the model should eventually forecast
endogenously. Thus it makes good sense to break down the forecasting of these payments into two
steps, first the constant-price or real value and then the current-price or nominal value.

Every equation in Models 1 through 4 is an identity, so our approach has earned its label as identity-
centered modeling. Essentially, we have tried to use identities to replace variables that are hard to
think about by others that are easier to grasp intuitively. Good use of identities is essential for good
modeling. This recognition of the central role of identities in modeling is in stark contrast to the way
that they are usually dismissed in econometric texts with the comment that an identity can be used to
eliminate a variable. Of course it can, and then one is left with a variable that is hard to think about
without the handle — the identity — that gives us an easy way to think about it.

Model 5, as already noted, introduces an extremely useful method of reducing the number of
exogenous variables by finding behavioral relations through regression analysis. Application of this
technique will occupy us through much of this book.




8
Model 1: The Pure Accounting Model

Table 1.2 shows the outline of the U.S. system of National Income and Product Accounts (NIPA) as
a sequence of equations.

The first equation defines Gross domestic product as

     gdp = c + v + x - m + g

where the variables have the meanings shown in the table. The variable names shown in the code
column are the ones used in the Quarterly Income and Product (Quip) data bank which goes with the
G software which we use in this book. Each equation after the first starts from the bottom line of the
preceding one and adds or subtracts (as shown in the operation column) the indicated items down to
the = sign, where the result is recorded and named. Thus, the second equation is

     gnp = gdp + irrow – iprow

We will refer to the first equation as the the "product side" of the accounts. The remaining equations
form the "income side" and may be less familiar. They show the transition from Gross domestic
product to Personal disposable income and Personal saving. (I follow the NIPA practice of
capitalizing only the first letter of the first word of each line in the accounts.)

Since these accounts will form the framework for much of our modeling, it is important that we
understand what they represent. The product-side equation for GDP defines it as total final product
of the economy. The word final here means that products used in making other products are not
counted. Thus, one farmer may grow corn and sell it to another, who feeds it to pigs, which he sells
to a packer, who sells the pork to a grocer, who sells it to the final consumer. Only the value of the
pork sold by the grocer goes into GDP. The word “Gross”, however, means that there has been no
subtraction for the wear out of capital.

We now need to follow carefully through the transition from GDP to Personal disposable income.
The first step, as already noted, is to convert from Gross domestic product to Gross national product.
The basic idea is that dividends or interest paid by a U.S. company to a foreign resident are part of
the domestic product of the U.S. but of the national product of the other country. More completely,
Income receipts from the rest of the world consist of receipts by U.S. residents of foreign interest
and dividends, of reinvested earnings of foreign affiliates of U.S. corporations, and of compensation
paid to U.S. residents by foreigners. Income payments to the rest of the world consist of payments
to foreign residents of U.S. interest and dividends, of reinvested earnings of U.S. affiliates of foreign
corporations, and of compensation paid to foreigners by U.S. residents.

Thus, to get GNP from GDP, we add U.S. rest of world income receipts and subtract U.S. income
payments.




                                                                                                         9
Table 1.2: U.S. National Accounts in Outline

op     code      Description
+      c         Personal consumption expenditures
+      v         Gross private domestic investment
+      x         Exports of goods and services
-      m         Imports of goods and services
+      g         Government consumption expenditures and gross investment
=      gdp       Gross domestic product

+      irrow     Income receipts from the rest of the world
-      iprow     Income payments to the rest of the world
=      gnp       Gross national product

-      ncca      Consumption of fixed capital
=      nnp       Net national product

-      nbctpn    Business current transfer payments (net)
+      pibtp     Business current transfer payments to persons
-      niprf     Corporate profits with capital consumption and inventory valuation adjustments
+      pidiv     Personal dividend income
-      netintm   Net interest and miscellaneous payments on assets
+      piint     Personal interest income
-      nwald     Wage accruals less disbursements
-      nsd       Statistical discrepancy
=      pibg      Personal income before government action

+      npctr     Personal current transfer receipts from government
-      nisurp    Current surplus of government enterprises
+      nisub     Subsidies (net)
-      nitpi     Taxes on production and imports
-      nconsi    Contributions for government social insurance
=      pi        Personal income

-      piptax    Personal current taxes
=      pidis     Disposable personal income

-      c         Personal consumption expenditure
-      piipcb    Personal interest payments
-      piptt     Personal current transfer payments
=      pisav     Personal savings




10
The next step is to subtract from GNP an estimate of capital consumption to get Net national
product. This estimate of Consumption of fixed capital is not depreciation as reported by firms in
their accounts and tax returns, for that depreciation is in historical prices, prices of the years in which
the assets were purchased. If prices have been increasing, the resulting number understates the
current cost of replacing what is wearing out. The Bureau of Economic Analysis (BEA) which
makes the NIPA therefore constructs a series of capital consumption in comparable prices by putting
investment in various physical assets into constant prices, say those of 2000, then applying
depreciation at rates appropriate to the different kinds of assets, then summing over assets invested
in various years, then summing over the various assets, and finally converting back to current year
prices.

In case you got lost in those words and really want to know what I was trying to say, here is the same
thing in a formula:
               n          t
       C t =∑ pi t   ∑      wi t− v i / p i 
              i=1        =−∞


where C(t) is Consumption of fixed capital in year t, n is the number of different types of physical
assets (buildings, trucks, machine tools, and so on), pi (t) is the price index of product i in year t, vi(τ)
is the investment in assets of type i in year τ, and wi(k) is the fraction of the initial value of an asset
of type i that depreciates in year k after it was bought. The difference between C(t) and the
depreciation reported for accounting purposes is called the Capital consumption adjustment
(CCAdj). If this difference is positive, as it usually is, then profits have been overstated in
conventional accounting, but they are corrected in the NIPA. The point of all this calculation is to
provide figures for Net national product and Corporate profits that are not improperly influenced by
past inflation or by modifying depreciation rules in the tax code.

Now we come to a big step, the conversion of Net national product to Personal income. We will take
it in two smaller steps. In the first, everything relates to the private economy, while in the second
everything is a government action. In the private-sector step, there are three smaller stages. The
first is:

- nbctpn        Business current transfer payments (net)
+ pibtb         Business current transfer payments to persons

Business current transfer payments (net) include legal settlements and theft losses. We first subtract
the entire amount and then add back in the portion which went to persons. The stolen case of
Bourbon is counted as income to the thief and, by the way, appears in Personal consumption
expenditure and thus in GDP.

The second stage is

-   niprf       Corporate profits with capital consumption adjustment and inventory valuation
                adjustment
+ pidiv         Dividend payments to persons

                                                                                                           11
Corporate profits, as such, do not go to Personal income; Dividends, however, do. Notice that the
profits here have been adjusted by the same capital consumption adjustment that was used in the
Capital consumption allowance. The net effect of this adjustment on Personal income is therefore
exactly zero. The inventory valuation adjustment is an adjustment for the change in the values of
inventory due to price changes. If a store is using “first in first out” inventory accounting and prices
have been increasing, then the store has been charging itself for inventory at less than the
replacement costs of the goods sold and thus exaggerating profits. This adjustment removes this
effect. Since the adjustment is also made to the amount of inventory change included in investment
on the product side of the accounts, the net effect of this adjustment on personal income is also
exactly zero.

Finally comes the puzzling pair

-    netintm Net interest and miscellaneous payments on assets
+    piint   Personal interest income.

Net interest is all interest paid by business less interest received by business. It is also all the interest
that is included in GDP. Interest paid by consumers on credit cards, automobile loans, or installment
credit is not counted as part of Personal consumption expenditure and therefore is not part of GDP.
(Do you think it should be?) Similarly, interest on the national debt is not part of Government
purchases of goods and services. (Should it be?) Interest paid by business -- and thus included in the
price of goods -- is, however, included in GDP. In particular, interest on home mortgages is
included because home owners have been converted into a fictional business in the NIPA; they rent
their homes to themselves and the imputed space rental value of these homes, including the interest
costs, is part of Personal consumption expenditure. This pair of lines, therefore, removes all interest
that is in GDP and adds back all the interest received by persons. The net effect is positive because
of the large interest payments of the government to persons.

The next item in the transition, Wage accruals less disbursements, is simply the difference between
the wages which have been earned and those which have been paid in the period. It is usually small
and attributable to the fact that many people are paid every week or every other week, so that the pay
periods do not correspond exactly to calendar quarters.

If we may, for the moment, skip the Statistical discrepancy, which should be zero conceptually, we
have reached Personal income before government action. This item is not in the official accounts,
but I find it a convenient point of reference. All of the transition from GDP to Personal disposable
income down to this point has been the result of events in the private economy; the rest of the way is
dominated by the government.

The first of the government actions, a huge one, is to add in Personal current transfer receipts from
government. These include Social Security and other government retirement programs, Medicare
and Medicaid payments, unemployment insurance, and all welfare programs. A small item, Current
surplus of government enterprises, is subtracted. These are part of net national product, but do not
go to persons. Then the Subsidies, such as those to farmers, are added in. Then the indirect taxes
(Taxes on production and imports) are removed. These are taxes such as property taxes on business

12
property or business franchise taxes or licenses. These charges are included in the prices of the
products which compose GDP, but they go to governments, not persons, so they must be removed
before we can get Personal income.

This would seem to me to be the logical place to draw a line and call the result Personal income.
The official definitions, however, remove one more item, Contributions for government social
insurance. These include Social Security contributions from both the employer and employee and
other similar programs. To me, they seem more akin to personal taxes than to the other previously
deducted items. In any event, once they are deducted we reach the official Personal income.

From it, deduction of Personal current taxes (mainly income and property taxes but also estate taxes
and nontaxes such as fines or admission to parks) leaves Disposable personal income. (I find it
peculiar that the nontaxes are not part of Personal consumption.) To reach Personal savings, we
must deduct not only Personal consumption expenditures but also Personal current transfer
payments, which include transfers to foreigners (mainly American residents sending money to
family in the Old Country) and transfers to government. We also must remove personal interest
payments, which are mostly interest paid by consumers to business. Here at last we have the interest
on credit cards and consumer loans which, as mentioned above, are not counted into Personal
consumption expenditure. But of course they are real outlays, so they must be removed before
reaching Personal saving by the last of the identities.

We have still not explained, however, the strange little item called Statistical discrepancy. It arises
because there are two different ways of measuring GDP. One is by measuring what is produced; the
other measures the income generated by producing it. The first, the product measure, is given by
gdp = c + v + x - m + g. The second, the income definition, begins with National income defined
conceptually as earned income and statistically as the sum

     +    nice      Compensation of employees
     +    niprop    Proprietors income with CCadj and IVA
     +    niren     Rental income with CCadj
     +    niprf     Corporate profits with CCAdj and IVA
     +    netintm   Net interest and miscellaneous payments on assets
     +    nitpi     Taxes on production and imports
     +    nisurp    Current surplus of government enterprises
     -    nisub     Subsidies
     +    nbctpn    Business current transfer payments (net)
     =    ni        National income.

To get from National income to Gross domestic product, we just need to add or subtract:

     +    ncca      Consumption of fixed capital
     +    nsd       Statistical discrepancy
     =    gnp       Gross national product
     +    iprow     Income payments to rest of world
     -    irrow     Income receipts to rest of world

                                                                                                     13
     =   gdp       Gross domestic product.

Now notice that this second, income-based route to GDP does not require estimating Personal
consumption expenditure, nor Gross private domestic investment, nor anything else in the product-
side definition of GDP. Conceptually, the two approaches should give exactly the same answer.
Will they in practice? You can bet they won’t. Many of the items in the accounts must be deduced
indirectly from other data. Consider just the problem of figuring out personal consumption of
gasoline. Suppose that you have reliable data on retail sales of gasoline, but how much goes to
vehicles being driven for business, and therefore not part of Personal consumption, and therefore not
part of GDP? There is certainly ample room for error in such estimates. The income side is no less
subject to error; just think of the incentives to avoid reporting proprietor income.

The difference between the two estimates of GDP is known as the Statistical discrepancy. The
statisticians making the accounts strive to keep it as close to zero as possible. The American
accountants, however, claim that they will not distort one of the components from what they believe
is their best estimate of it just to get a low Statistical discrepancy. In most other countries, the
accountants adjust the components until the discrepancy is eliminated. In the U.S. NIPA, National
income and Personal income are based on the income measurement, while Gross domestic product is
based on the product measurement. The Statistical discrepancy slips between the two so that the
tables balance.

I must confess to you that I find the working out of the concepts of the NIPA and the devising of
ways to estimate all the components the most impressive and important contribution that economics
has made to understanding the world in which we live. Today, we take these accounts more or less
for granted, but it is well to remember that in 1930 they did not exist.

We can now summarize this first model in the following master file which shows the actual
commands to the software we use to compute this model.


     # Master File for Model 1
     # 31 variables, 8 identities, 23 exogenous variables
     # Gross domestic product
     f gdp = c + v + x - m + g
     # Gross national product
     f gnp = gdp + irrow - iprow
     # Net national product
     f nnp = gnp - ncca
     # Personal income before government action
     f pibg = nnp - nbctpn + pibtp - niprf + pidiv - netintm + piint - nwald -nsd
     # Personal income
     f pi = pibg + pigsb – nisurp + nisub - nitpi - nconsi
     # Personal disposable income
     f pidis = pi - piptax
     # Personal savings
     f pisav = pidis - c - piipcb - piptt
     # Addendum: National income
     f ni = nnp - nsd
     end




14
     # Graphical checks of the identities
     ti GDP
     gr gdp a.gdp
     ti Personal income
     gr pi a.pi
     ti Personal savings
     gr pisav a.pisav
     ti National income
     gr ni a.ni


Lines that begin with a # are just comments for the benefit of the human reader and are ignored by
the computer. Lines that begin with an “f” form the variable on the left by the expression on the
right. The “end” command signals the end of the master file for the model building program. Lines
below it do not go into the model but have another function.

The lines below the “end” command provide graphical checks of the correctness of the identities. If
you start G7 and on the main menu click Model | Master, you will see this master file in the green
edit window to the right. Normally, you do not “run” a master file, but with this model of only
identities it makes sense to do so precisely to check the identities. Click the Run button on the main
menu of the green editor, and all the commands will be executed by the G7 program. The variables
on the left of the “f” commands will be formed and placed in a workspace bank. When G7 needs a
variable, it looks first in this workspace; if it finds no variable of the required name, it looks in the
assigned bank “a”. When G7 has been started in the \ami directory, the bank assigned as “a” is Quip,
the Quarterly Income and Product bank. Thus, when this file has been run through G7, the variables
created by the “f” commands will be in the workspace bank. If the identities are correct, variables
like gdp, gnp, pi, pisav, and ni should have the same values in both banks. To check that they do –
that is, that we have made no mistake in writing down the identities – we can graph what should be
the same series from the two banks. We can force G7 to go to assigned bank “a” for a variable by
putting an a. in front of the variable name. Thus, the command
          gr gdp a.gdp

will graph gdp from the workspace in the first (red) line and gdp from the assigned bank “a” with the
second (blue) line. If our identities are correct, the two lines will coincide. In the present case,
passing this check is relatively simple. To see how the test works, however, you may want to
deliberately mess up one of the identities, “run” the false Master file, and note how the graphs reveal
that something is wrong. Be sure to return the Master file to its correct state and run it again.

There is absolutely nothing in this model to object to. It will reproduce the historical data exactly.
And yet it is a rather strange model. If we increase government purchases, g, by $100 billion,
Personal income will increase by $100 billion, Personal consumption will be unchanged, and
Personal savings will increase by $100 billion. That is surely an implausible result. Are consumers
utterly indifferent to their income in deciding how much to consume? Certainly not. That
observation leads us to Model 2. Because we can completely analyze Model 1 so easily, we will not
actually build and run it. That we will do first with Model 2.

Before moving to it, however, I should add that if perhaps you feel these NIPA accounts are
somewhat complicated, you should see those used by nearly all other countries. They use a structure

                                                                                                       15
known as the System of National Accounts (SNA) developed by the United Nations and other
international organizations. In the SNA, the basic pure-identity model requires nearly four times as
many variables and three times as many identities as does our model above. In models based on the
SNA, the identity checks are very important. I have on several occasions spent several days getting
identities correct in models based on these accounts.


Model 2. A Behavioral Ratio for Personal Consumption

We have noted the peculiarity in Model 1 that consumption is independent of Personal disposable
income. Let us introduce a ratio between them, a ratio that will be constant in any one period but
which may vary over time. Because it expresses the behavior of consumers, it is natural to call it a
behavioral ratio. The command in our software for forming this exogenous ratio is
     fex cBR = c/pidis

The “fex” stands for “form the exogenous variable”. It is important to realize that the “fex”
command forms the variable on the left and puts it into the model’s data bank, but it does not put the
equation into the model. In this respect, it differs fundamentally from the “f” command which not
only forms the variable but also puts the equation into the model.

Once we have formed the behavioral ratio, we use it in the equation
     f c = cBR*pidis

The Master file is changed at the end. As far down as the definition of Personal disposable income,
it is unchanged. Starting from that point, the end of the new master file for Model 2 is:
     # Personal disposable income
     f pidis = pi – piptax
     # Personal consumption expenditure
     fex cBR = c/pidis
     f c = cBR*pidis
     # Personal savings
     f pisav = pidis - c - piipcb – piptt
     # Addendum: National income
     f ni = nnp - nsd
     check c .2
     end


This procedure of introducing cBR at first looks entirely circular. You probably wonder, what is the
purpose of introducing cBR if we only use it to derive c? c was used to derive cBR in the first place!
However, it is important to remember that this new variable, cBR, is exogenous to the model, and
allows us further insight into the workings of our model economy. We can determine cBR for all
years for which we already have economic data. For any future years, we will have to supply it,
which you will see should not be very hard. Having cBR as an exogenous variable allows us to run
counter-historical simulations which ask the question: “What if the consumption ratio had been 5%
higher in all years than it was observed to be?”


16
(From here on, we omit the commands to check the identities.) The key new commands, lines 4 and
5 and the next to last, have been set in boldface type. Notice that Personal consumption
expenditure, c, has now become an endogenous variable and that we have introduced a new
exogenous variable, cBR, so the number of exogenous variables has not been changed. Furthermore,
the model remains tautological in the sense that -- given the historical values of the exogenous
variables -- it will reproduce exactly the historical values of the endogenous variables.

If, however, we do a counter-historical simulation by running the model over the historical period
but with one (or more) of the exogenous variables changed from the historical values, then we get a
big difference from Model 1. For example, we can increase the value of Government purchases, g,
by 20 in all periods. The ratio of the increase in gdp to this increase in g is called the “gdp multiplier
of g” or just the “GDP/G multiplier”. For Model 1, that ratio was exactly 1.0 for all periods. The
figure below shows its values for Model 2.

                                  The Government Multiplier in Model 2
                       30.8




                       18.7




                       6.6

                    1980          1985       1990         1995        2000         2005
                           mult



The key to understanding the big difference between Model 2 and Model 1 is the next to last line of
the master file, the check c .2. Note that a value of c was assumed at the top of the master file for the
calculation of gdp. At the end, just above the “check” command, the value of c implied by all the
equations is calculated. Will it be the same as the value used at the top? If all of the other variables
are at their historical values, then the answer is Yes. But if one of them, say g, has been changed to
some other value, then the answer on the first try is No. Our “check” command checks to see if the
difference between the initial and the implied value is less than .2. If so, it allows the model to go on
to the next period; if not, it sends the calculations back to the top of the master file, but this time with
the value of c implied on the previous pass as the value assumed at the top. This method of solving
the equations of the model, which is known as Seidel iteration, is not certain to converge. In
practice, however, it almost always does so in a well formulated model.


                                                                                                         17
Clearly the economic properties of Model 2 are widely different from those of Model 1. We now
have, almost certainly, too much multiplier, for we have taken no account of the fact that taxes
should also go up as income rises. That and other matters will be handled in Model 3.

Before leaving Model 2, however, we should perhaps ask why there has been such a rise over time in
the multiplier. You may perhaps recall from a macroeconomic course that the multiplier should be
one over the savings rate. In our terms that would be 1/(1 - cBR). In fact, calculating the multiplier
by this formula gives exactly the same graph as that shown above. Thus, the cause of the rise in the
multiplier is the decline in the savings ratio, or more properly, the rise in the ratio of consumption to
income.

                                                Exercise

1.2. Build and run Model 2. Run it first with all exogenous variables at their historical values. Then
run it again with one or more of them changed. Compare the results graphically. Compute and graph
the multiplier. Here are the steps:

1.   Start the G7 program by double-clicking its icon. If working on your own machine, you will
        first need to install the software which accompanies this book, including the Borland C++
        free compiler. Follow the directions in the readme.txt file. If you are working on a network
        for a course, your instructor will explain how to find it. G immediately asks you to select the
        folder and configuration file. You want to “look in” c:\ami and the file you want to “open” is
        g.cfg.

2.   Prepare the Master file. On the main menu bar, click Model | Master. (The use of the | symbol
        will be our shorthand way to say: click Model and then on the drop-down menu, click
        Master.) An editor window opens in the upper right corner of your screen and displays the
        Master file of Model 1. Do “File | Save as ..” to save it as Master1 in case you need it again.
        Edit the file to transform it into the Master file of Model 2. The editor works pretty much
        the way that any Windows-based word processor works. Again click “File | Save as ..” on
        the Editor’s menu and save the file as Master (not Master2).

3.   Build the model. On the main menu of the blue window to the left, click Model | Build. This
       simple step calls a console application program to build an executable model from the
       current version of the Master file. If all goes well, all you have to do is tap a key at the end
       of the operation to return to G7. This step, it must be emphasized, works from the file
       presently called Master – not Master2 , or Master3, or anything else.

4.   Run a historical simulation. On the main menu, click Model | Run. A window will request the
       starting date, ending date, results file name, and fix file name. To say that you want the
       model to run from the first quarter of 1980 to the third quarter of 2002, enter the dates as
       follows: 1980.1 and 2007.3 . (Run your model up to the most recent period for which you
       have data. It may be more recent that the example I have used.) Let us call the results file
       “histsim2” (for “historical simulation of model 2”) and use “rhoadj” for the fix file. Exactly
       what that strange name means we will explain in due course. Then click OK. You should

18
        get a black screen informing you that you are running the AMI model and giving you a ]
        prompt. Respond with “ti xxx” where xxx is up to 80 characters giving your title for this run
        of the model. Take the time to give a meaningful title such as “Historical Simulation of
        Model 2.” You then get the ] command again; reply with “run”. The model runs, some
        printout goes by too fast to read, and the program stops with the message “Tap any key to
        continue.” This pause allows you to see error messages. With good luck, there will be none.

5.   Graph the results of the historical simulation. First you must adapt the hist.shw file to the
       current model. Open it in the green editor window. Commands at the top of the file open
       the banks from which the data will be taken. In the present case, we should make them
            bank histsim2     b
            bank bws c

6.   On the editor’s main menu, click File | Save as ... and save the file as hist2.shw. Then click Run
       on the editor’s menu. A graph and a run control panel should appear. Click the “continue”
       button on this panel, or tap the letter ‘c’, to view all the graphs, one after another. Since the
       model is tautological, all the graphs should show only one line, though if you look closely
       you will see that there are red crosses inside the blue squares. On the control panel, you can
       click the “Max” button to maximize the size of the graph. Click the “Normal” button to
       return the graph to its size before hitting the “Max” button.

7.   Now to make a run with government purchases of goods and services, g, increased by $100
       billion, repeat step 4 but call the results “highgov2” and at the ] prompt give a title like “ti
       Government increased by $100 billion.” Then, still at the ] prompt, do
            type g
            update g +100
            type g
          You should see that the values of g have all been increased by 100. Then give the “run”
          command. You are doing a counter-historical simulation.

8.   Now make graphs comparing the historical simulation, which is in the bank histsim2, with the
       counter-historical simulation, which is in the bank highgov2. Edit the file comp.shw. You
       can do “ed comp.shw” in the white command box, or you can click Editor on the main menu
       and the Open on the Editor’s menu and open comp.shw Window’s style. Make it begin with
       the commands
            bank highgov2 b
            bank histsim2 c


        At the end of this file, you should put the commands to compute the multiplier and graph it.
        Here they are:
            f mult = (b.gdp - c.gdp)/(b.g - c.g)
            ti The Government Multiplier in Model 2
            gname mult
            gdates 1980.2 2007.3
            gr mult
            gdates 1980.1 2007.3


                                                                                                          19
          The “gdates” commands control the dates for the beginning and end of the graphs. The
          mult variable will be zero in the base year of the model run, for the model is not computed
          for that base period. Hence, in order to graph only the non-zero values of the multiplier,
          we begin the graphs in the second period of the run. Adapt the second date in these
          commands to the most recent data in your data bank. After this use, the second “gdates”
          command restores the dates to their normal values. Click the File | Save as ... on the main
          menu of the green editor window. Save the file as comp2.shw, that is, as a file adapted to
          making the comparisons for Model 2. You may then “run” the file by clicking “Run” on
          the main menu of the green editor window. That will save the file and execute the
          commands in it in G. You should see the graphs go by as you click the Continue button on
          the left of control panel which appears along with the graphs.

9.   Now we want to save the graphs created in step 7 so that they can be imported into a word
       processor. Look again at the file comp2.shw in the editor. You will see triplets of lines like
       this:
               ti gdp — Gross Domestic Product
               gname gdp
               gr b.gdp c.gdp
       The second line is what interests us here. It gives the filename of the file into which the
       graph will be saved if you click the “Save” button on the run control panel. To make the
       graphs again, you can either repeat the command “add comp2.shw” or, since you already
       have comp2.shw in the editor, you can click “Run” on the Editor menu. On several of the
       graphs, click the “Save” button on the run control panel. Let us say that you saved the gdp
       graph. Actually, you created a file named gdp.wmf, the “wmf” standing for Windows
       Metafile. Start your word processor and bring in this file. In WordPerfect the command is
       Insert | File. In Word and Open Office it is Insert | Picture. Drag the little square handles on
       the graph to size as you like.


Model 3. More Behavioral Ratios and Shortcutting Exogenous Variables

In the preface, I said that models are “serious business” because they are playthings. A good
plaything should be simple enough to make playing with it fun. Model 2, while clearly a step in the
right direction, still has an ungainly number of exogenous variables that have to be set whenever we
want to run it. Furthermore, it still ignores a number of important behavioral relations. Model 3
helps with both those problems. It puts in several more important behavioral relations and
eliminates a number of exogenous variables of secondary interest. In particular, there are no less
than eleven exogenous variables between gdp and pibg. If we are prepared for the moment to ignore
the distinctions among them, then we can make the entire transition in one step by introducing an
exogenous behavioral relation pibg and gdp. Model 3 also adds a behavioral relation between
Indirect taxes and GDP. It drops the distinction between “contributions for social security” and
“personal taxes” and specifies the sum of the two as a behavioral ratio to a pibg less the Indirect
taxes. At the same time, it adds important behavioral ratios between:


20
    Imports and private demand
    Personal savings and Personal disposable income
    Interest paid by consumers to business and Personal disposable income
    Personal transfers to foreigners and Personal disposable income.

The last three replace the single ratio between Personal disposable income and Personal
consumption expenditure which was introduced in Model 3. Here is the new master file.
    # Master 3
    # 21 variables, 11 exogenous (not counting time)
    # 7 behavioral ratios for m, pibg, nitpils, ptax, piipcb, piptt, pisav
    # v, x, g, npctr remain exogenous
    checkdup y
    fex time = time
    # Behavioral ratio for imports
    fex mBR = m/(c+v+x)
    f m = mBR*(c+v+x)

    # Gross domestic product
    f gdp = c + v + x - m + g

    # Shortcut to Personal income before government action
    fex pibgBR = (gdp + irrow - iprow - ncca - nbctpn - nsd - niprf +
              pidiv - netintm + piint - nwald + pibtp)/gdp
    f pibg = pibgBR*gdp

    # Calculate personal tax base
    fex nitpilsBR = (nitpi - nisub)/gdp
    f nitpils = nitpilsBR*gdp
    # ptaxbase: Personal tax base
    f ptaxbase = pibg - nitpils

    # ptaxBR is personal tax rate including Social Security
    fex ptaxBR = (piptax + nconsi)/ptaxbase
    f ptax = ptaxBR*ptaxbase

    # Personal disposable income
    f pidis = pibg - nitpils - ptax + npctr

    # Behavioral ratios for piipcb, piptt, and pisav
    fex piipcbBR = piipcb/pidis
    f piipcb = piipcbBR*pidis
    fex pipttBR = piptt/pidis
    f piptt = pipttBR*pidis
    fex pisavBR = pisav/pidis
    f pisav = pisavBR*pidis

    # Implied consumption
    f c = pidis - piipcb - piptt - pisav

    check c .2
    end


In Model 3, only 4 of the original 23 exogenous variables remain, namely v, x, g, and npctr. Seven
behavioral ratios, however, have been added, and one may well wonder how stable or how hard to
forecast they are. Above are graphs of them that should help answer that question.


                                                                                                     21
                                     mBR -- Import Ratio                                                pibgBR - PIBG Ratio
        0.18                                                               0.90


                                                                           0.89

        0.16

                                                                           0.88



        0.14                                                               0.87



                                                                           0.86

        0.12

                                                                           0.85


        0.10                                                               0.84

      1980               1985         1990         1995    2000   2005   1980              1985          1990          1995   2000   2005
             mBR                                                                pibgBR

                                             Tax Rates                                    Use Rates of Personal Disposable Income
        0.30                                                               0.12


                                                                           0.10



        0.20                                                               0.08



                                                                           0.06



        0.10                                                               0.04



                                                                           0.02


        0.00                                                               0.00

      1980               1985         1990         1995    2000   2005   1980              1985          1990          1995   2000   2005
             nitpilsBR      ptaxBR                                              pisavBR       pipttBR       piipcbBR




The scales have been chosen so that the variations in the variables can be seen. The PIBG ratio has
remained surprisingly stable between .84 and .90; the import ratio is more volatile between .10 and .
18 with a distinct tendency to rise. The indirect tax rate (nitpilsBR) has remained stable throughout
the period shown. The personal tax shows more variation. The Reagan tax cuts produced the slight
dip in the personal tax rate between 1981 and 1984; after 1984, they began to crawl back up. The
rise since 1995 reflect various “revenue enhancements” by the Clinton administration. Finally, the
dip in the tax rate starting in 2001 reflects the Bush tax cuts, and a weakening economy. Clearly the
most dramatic movement is in the Personal savings rate. This collapse of personal savings is a
matter into which we must look closely as we develop the model of the economy. With the
exception of the savings rate, it would seem possible to make pretty good guesses about the future
values of these ratios for a few years ahead.

The first game to play with a model, of course, is to calculate its GDP/G multiplier. The result is
shown in the graph below.




22
                                   The Government Multiplier in Model 3
                        2.04




                        1.97




                        1.89

                     1980          1985     1990        1995    2000      2005
                            mult




The values hover around 1.90, a value typical for a purely Keynesian model such as we have here.
When we account for inflation and monetary effects, the value should come down further.

                                                   Exercise

1.3. Bring your Master file up to the level of model 3. Build and run the model in both historical and
counter-historical simulation. How does it respond to the increase of $100 billion in government
purchases? Proceed as in the previous exercise; be sure to include a graph of the multiplier. You do
not have a file that automatically makes the graphs shown above, but with what you now know, you
should have no trouble making them. Remember that, after building a model, all variables in the
model are in the bank called bws.


Model 4. Seeing Past Inflation

So far, all of our variables expressed in dollars have been in nominal terms, that is, in terms of the
dollars actually spent. We have made no attempt to account for inflation. But anyone old enough to
remember can tell you that forty years ago (1967) a dollar would buy a meal that today costs six or
more. Consequently, these nominal or current dollar measures greatly overstate the growth that has
occurred in the economy. The makers of the NIPA have, therefore, produced another set of figures
for the product side of the accounts by expressing each item -- consumption, investment, exports,
imports, and government purchases -- in the prices of one particular, base year. Presently (2007),
that base year is 2000, but the base year is shifted forward every five years. These accounts are said
to be in constant dollars, or constant prices, or real terms. These are all equivalent expressions.

As long as one is dealing with products which are in common use over the period studied, there is no


                                                                                                    23
great conceptual problem in how to measure the change in an item’s price. For example, there is no
great problem in putting the 1982 consumption of milk or jeans into 2000 prices. But what about the
2000 price of a 1982 computer which was no longer made by 2000?

In fact, this very question of the pricing of computers has far-reaching effects in the U.S. NIPA. In
the mid 1980's, BEA adopted a method known as “Hedonic” price indexes for computers, which it
has extended to some other high-tech items. It looked at a variety of computers in, say, both 2005
and 2006 and recorded price and physical characteristics such as speed, RAM size, and hard disk
size. By doing a regression (by methods we shall study closely in chapter 2) of price on the physical
characteristics of the 2005 machines, an equation for the 2005 price as a function of physical
characteristics could be determined. Coming then to the 2006 models, this function could be applied
to say what their 2005 price would have been. By comparison with the actual 2006 prices, a price
index between the two years can be deduced. The figure on the left below shows the resulting price
index for computers and that for all other equipment. The figure on the right shows the annual
percent change in computer prices -- note that the numbers are all negative.



               Prices of Computers and Other Equipment                          Rate of Change of Computer Prices
                              2000 = 1.0                                                  Percent per Year
  3.00                                                            -0.0


                                                                  -5.0


                                                                 -10.0
  2.00

                                                                 -15.0


                                                                 -20.0

  1.00
                                                                 -25.0


                                                                 -30.0

  0.00                                                           -35.0

1985              1990        1995         2000          2005   1985            1990        1995             2000   2005
     compdef      vfnreD                                            grcompdef




The contrast between the treatment of high-tech items and everything else is stunning and, I believe,
problematic. This method of evaluating computers is not the only possibility. For example, one
might evaluate the marginal product of a computer by assuming that various factors of production
are paid their marginal product. When I first began programming computers in 1959, the computer
cost $60 per hour and my wage was about $2.00 per hour, so the marginal product of that computer
was about 30 times my marginal product. The machine on which I am typing these words is at least
a thousand times more powerful than that machine, but it costs less than one percent of what I cost
per hour. If I generously suppose that my marginal product has not declined over the intervening
years, that Univac-1 in 1959 must have had a marginal product about 3000 times that of the machine
in front of me now — not a thousandth of it! Thus, there seems to me to be no compelling reason to
use the BEA method of deflation; and, indeed, considerable reason to doubt that it is even of the
right order of magnitude. Consider another example. In the 1970's, many universities and
companies were equipped with usually one mainframe computer. It usually took up the space of an

24
entire room, and surely its power is dwarfed by today's personal machines. However, these
mainframes still computed an enormous amount of data and information for whomever would line
up. The BEA deflator takes these extremely important machines and writes off them and their
contributions to GDP as nearly nothing.

The use of the BEA computer and other high-tech deflators affect substantially the rate of growth of
real GDP. To prevent these effects from turning the accounts into nonsense, the BEA has offset the
effects of the Hedonic indexes by also adopting a non-linear procedure known as “chain weighting.”
For each pair of adjacent years, say 2005 and 2006, the real growth ratio of each component of GDP
is computed in two ways. In the first, all items are priced with the prices of the first year; in the
second, all items are priced with the prices of the second. We may call the first index A and the
second B. The geometric mean of the two,  AB is then taken as the real growth ratio of that
component between those two adjacent years. If the growth between non-adjacent years, say 2003
and 2006, is needed, it is taken as the product of all the year-to-year growth ratios between the two
years, that is, the year-to-year ratios are chained. To get GDP or a component in prices of a base
year, say 2000, one just moves the 2000 nominal GDP forward or backward by these year-to-year
growth rates.

The official constant-price series are found in the QUIP data bank with names derived from the name
of the current dollar series by adding a $. Thus, the official series for real GDP is gdp$.

The use of chaining means that it is never necessary to evaluate the output of computers (or other
goods) in prices more than one year distant from the year in which they were made. That is certainly
an advantage, but chaining also has a major disadvantage. Namely, total real GDP, say for 2007 in
prices of 2000, is not the sum of the components of real GDP. The same is true with the
components. For example, the official 1960 value of fixed investment (vf$) in 2000 prices is 27
percent smaller than the sum of its three major components: residential construction (vfr$), non-
residential construction (vfnrs$), and non-residential equipment (vfnre$). It will clearly be a
nuisance in modeling if a total is not the sum of its parts.

Because I have serious reservations, as already explained, about the computer and other high-tech
deflators and do not wish to be plagued with totals that are not the sum of their parts, I am going to
adopt a radical simplification for our modeling. I am going to use the deflator for personal
consumption expenditures on food as the deflator for all series in the national accounts. I choose
food because it is important and I tend to trust the methods of making indexes for food prices. I also
notice that when I need a subjective price index to put some dollar amount from twenty or thirty
years ago into current prices, I tend to try to recall food prices. Remarkably, the trend of food prices
and that of the GDP deflator computed with Hedonic indexes and chain weighting are very similar,
as seen below. The food prices have, however, been somewhat more volatile, as shown in the right
graph. In both graphs, the heavier, smoother line is the official GDP deflator. GDP and GDP
product-side components deflated by the Food deflator have names in the QUIP databank derived by
adding R (for “Real”) to the variables name in current prices.

With the question of the computer deflator settled, we can turn to the proper subject of Model 4, the
accounts in real terms. There are many reasons for wanting accounts in real terms. If we want to

                                                                                                     25
                      The Official GDP and Food Deflators               Inflation in the Official GDP and Food Deflators
                                    2000 = 1.0                                                  2000 = 1.0
         1.20                                                     15.0



         1.00


                                                                  10.0
         0.80




         0.60
                                                                  5.0


         0.40


         0.20                                                     0.0

     1960              1970       1980           1990   2000   1960             1970          1980           1990   2000
            cnfoodD       gdpD                                        GDPinfl      Foodinfl

use GDP or Personal consumption expenditure as an indicator of welfare, it is certainly the measure
in real terms that is relevant. If we want to relate employment to the NIPA, we clearly need
measures in real terms. If we want to tie investment to growth in output, it is only real growth which
is relevant. Also, forecasting exogenous variables in real terms is generally easier than in nominal
terms. The model should make the conversion from real to nominal for us.

In the official accounts, each item on the real side has its own deflator, and there are no deflators on
the income side except for an addendum item which gives Personal disposable income deflated by
the Personal consumption expenditure deflator. Since we are using accounts in constant “food
dollars,” all items have the same deflator. We will make the GDP deflator an exogenous behavioral
ratio, and set the consumption deflator relative to it. All the exogenous dollar amounts will be
exogenous in real terms. Here is the master file of Model 4.
     #   Master 4: Product side in real terms
     #   13 exogenous (not counting time)
     #   7 behavioral ratios for m, pibg, nitpils, ptax, piipcb, piptt, and pisav.
     #   gdpD exogenous
     #   vR, xR, gR, npctrR exogenous in constant dollars

     checkdup y
     fex time = time
     # Behavioral ratio for imports
     fex mBR = mR/(cR+vR+xR)
     f mR = mBR*(cR+vR+xR)

     # Real Gross domestic product
     f gdpR = cR + vR + xR - mR + gR

     # Current price values
     f m = mR*gdpD
     f v = vR*gdpD

     # Nominal GDP
     f gdp = gdpR*gdpD

     # Shortcut to Personal income before government action
     fex pibgBR = (gdp + irrow - iprow - ncca - nbctpn - nsd - niprf +
               pidiv - netintm + piint - nwald + pibtp)/gdp
     f pibg = pibgBR*gdp



26
    # Calculate personal tax base
    fex nitpilsBR = (nitpi - nisub)/gdp
    f nitpils = nitpilsBR*gdp
    # ptaxbase: Personal tax base
    f ptaxbase = pibg - nitpils

    # ptaxBR is personal tax rate including Social Security
    fex ptaxBR = (piptax + nconsi)/ptaxbase
    f ptax = ptaxBR*ptaxbase

    # Government transfer payments to persons, exogenous in real terms
    fex npctrR = npctr/gdpD
    f npctr = npctrR*gdpD

    # Personal disposable income
    f pidis = pibg - nitpils - ptax + npctr

    # Behavioral ratios for piipcb, piptt, and pisav
    fex piipcbBR = piipcb/pidis
    f piipcb = piipcbBR*pidis
    fex pipttBR = piptt/pidis
    f piptt = pipttBR*pidis
    fex pisavBR = pisav/pidis
    f pisav = pisavBR*pidis

    # Implied consumption
    f c = pidis - piipcb - piptt - pisav
    # convert to real terms
    f cR = c/gdpD
    check cR .2
    end


It is natural to be asking at this point, “Very well, as long as we know all the exogenous variables
and behavioral ratios exactly, these models enable us to calculate GDP exactly. But if we are
forecasting, we don’t know them. How good are the models if we must make rough guesses for the
exogenous variables?” A partial answer may be had by seeing how well the models would have done
in the past had we used constant values for all the behavioral ratios. I looked over the values of
each behavioral ratio and picked the following typical values: pibgBR = .87, mBR = .13, nitpilsBR
= .07; ptaxBR = .217, piipcbBR = .025; pipttBR = .006, pisavBR = .055. When the model was run
with these values from 1980.1 to 2007.3, it produced the simulation shown by the line marked with a
+ in the graph below.




                                                                                                 27
                                                             gdpR - Real GDP
                                                    Actual v. Simulation with All BRs Constant
                              12346




                               8726




                               5106

                             1980            1985           1990           1995          2000    2005
                                    b.gdpR      a.gdpR




The model did quite well up through 2000 but by 2003 it went racing off beyond the economy’s
actual course. In fact, exports were rising considerably, but the rising exports went with a rising
share of imports. The neglect of this rise (we kept mBR constant) is responsible for the runaway
growth in the simulation. My fundamental impression from this graph, however, is that it is possible
to model the real growth of the economy surprisingly well given just investment, exports,
government purchases and transfers. Of course, explaining the movements in the major behavioral
ratios would improve the simulation. A method for doing so is introduced in Model 5.

                                                                   Exercise

1.4. Bring your model up to the level of Model 4. Run the counter-historical simulation with gR
increased by 100. Using comp.shw, make graphs of the effects on the new, real-terms variables such
as gdpR, cR, and so on. Graph the multiplier.


Model 5. Modeling Behavior with Regression

So far, we have been able to replace one exogenous variable with another which is easier to
comprehend or forecast. But we have not reduced the number of exogenous variables. With Model
5, we will strike investment from the list of exogenous variables but keep it in the model, now as an
endogenous variable. We will do so by fitting two equations by the method of least squares, one for
vfR, fixed investment, and one for viR, inventory investment. We will also replace the rather volatile
behavioral ratio for imports, mR, with an estimated equation. We will, in the process, reduce the
number of exogenous variables by two.

The fixed investment equation explains vfR as a linear function of estimated replacement and lagged
values of changes in real GDP. The commands to the computer, using the G7 software, are put into a
file, called vfR.reg in this case, and this file is then executed by G7. The contents of vfR.reg are just
     # vfR.reg - Regression to explain fixed investment
     catch vfR.cat
     save vfR.sav


28
     lim 1975.1 2007.3 2007.3
     ti Gross Private Domestic Fixed Investment
     f d = gdpR - gdpR[1]
     f replace = .05*@cum(stockf,vfR[4],.05)
     r vfR = replace, d[1], d[2],d[3],d[4],d[5],
        d[6],d[7],d[8],d[9],d[10],d[11],d[12]
     save off
     gr *
     catch off

The results of the regression are a graph showing how well the equation fits and two files with the
results. One of these files, created because of the command in the first line, is called the catch file
and is intended to be looked at by humans and included in books and papers. The other, created
because of the command in the second line and called the save file, is intended to be read by the
computer in making a model. The first two commands in the vfR.reg file just name the catch and
save files. The third, with the lim command, specifies the period over which the equation is to be fit;
the fourth supplies a title for the graph and the catch file. Then comes the definition of the variable
d as the difference between one period’s value of gdpR and the previous period’s value. (The []
notation in G7 is to indicate lags of variables.) The next line defines the stock of fixed capital,
stockf, by cumulating investment with a depreciation rate of 5 percent per quarter and the sets
replacement (called replace) equal to 5 percent of the stock. We will explain the @cum function
further in a later chapter; suffice it to say here that it does just what an accountant using declining
balance depreciation does. Finally, we come to the command that we have been leading up to, the
one that fits a linear function of the variables listed on the right side of the = sign to the variable on
the left. This fitting is often called linear regression; hence the r as the name of the command. We
will look carefully at how this fitting is done in the next chapter. The remaining commands just
draw the graph of the fit and close the save and catch files. Here is the graph for our equation.
                                              Gross Private Domestic Fixed Investment
                            1887




                            1206




                             525

                          1975               1980            1985   1990   1995   2000   2005
                                 Predicted          Actual




The fit is clearly quite close, almost uncomfortably close if you consider how simple the equation is.
Notice that it does not use the interest rate or any other financial or monetary variable. In producing
a fit this close without such a variable, the computer seems not to have realized that the whole of
macroeconomics pivots on the interest rate in the IS curve. Such a heretical computer should

                                                                                                       29
probably be burned at the stake.

The results in the “catch” file are:
     save vfR.sav
     lim 1975.1 2007.3 2007.3
     ti Gross Private Domestic Fixed Investment
     f d = gdpR - gdpR[1]
     f replace = .05*@cum(stockf,vfR[4],.05)
     r vfR = replace, d[1], d[2],d[3],d[4],d[5],
         d[6],d[7],d[8],d[9],d[10],d[11],d[12]
     :                     Gross Private Domestic Fixed Investment
        SEE    =     75.26 RSQ   = 0.9514 RHO =     0.96 Obser = 131 from 1975.100
        SEE+1 =      21.82 RBSQ = 0.9460 DW =       0.08 DoFree = 117 to    2007.300
        MAPE =        6.26
          Variable name           Reg-Coef Mexval Elas       NorRes    Mean    Beta
        0 vfR                   - - - - - - - - - - - - - - - - -    1186.71 - - -
        1 intercept              126.66241     11.7    0.11   20.58      1.00
        2 replace                  0.86737   200.5     0.72    2.17   980.41 0.781
        3 d[1]                     0.41253      2.7    0.02    1.99    55.06 0.059
        4 d[2]                     0.44480      3.0    0.02    1.83    53.88 0.065
        5 d[3]                     0.42470      2.6    0.02    1.66    53.89 0.062
        6 d[4]                     0.43960      2.9    0.02    1.51    53.10 0.065
        7 d[5]                     0.46876      3.4    0.02    1.35    53.27 0.069
        8 d[6]                     0.35820      2.0    0.02    1.25    52.59 0.053
        9 d[7]                     0.29141      1.3    0.01    1.18    51.92 0.043
       10 d[8]                     0.22812      0.8    0.01    1.13    52.44 0.034
       11 d[9]                     0.30990      1.4    0.01    1.06    52.09 0.045
       12 d[10]                    0.25860      1.0    0.01    1.02    51.82 0.038
       13 d[11]                    0.18757      0.5    0.01    1.01    51.93 0.028
       14 d[12]                    0.12654      0.3    0.01    1.00    51.91 0.019

     save off
     gr *
     catch off


The column headed "Reg Coef" shows the regression coefficients, that is, the values of the
parameters in the equations. The equation which will enter the model is
vfR =    126.66241    + 0.86737*replace + 0.41253*d[1] + 0.44480*d[2] +
         0.42470*d[3] + 0.43960*d[4] + 0.46876*d[5] + 0.35820*d[6] +
         0.29141*d[7] + 0.22812*d[8] + 0.30990*d[9] + 0.25860*d[10] +
         0.18757*d[11] + 0.12654*d[12]


The terms used in these numerical displays will be explained in detail in the next chapter; but, in
case you have already studied statistics and regression, here is a brief explanation of the terms for
reference. (Do not worry if some or all of them are unfamiliar to you; all will be made clear in due
course.)

     In the top panel:




30
Statistic Name           Description
SEE                      Standard error of estimate, or the square root of the average of the squares of the misses or
                         "residuals" of the equation. (Not adjusted for degrees of freedom.)
RSQ                      (pronounced r-square) = Coefficient of multiple determination.
RHO                      (Greek rho) = Autocorrelation coefficient of the residuals.
Obser                    Number of observations.
SEE+1                    The SEE for forecasts one period ahead using rho adjustment.
RBSQ                     (pronounced r-bar-square) = Coefficient of multiple determination adjusted for degrees of
                         freedom.
DW                       Durbin-Watson statistic; contains same information as does RHO.
DoFree                   Degrees of freedom = Obser minus number of independent variables.
from                     Starting date of the regression
       to                Ending date of the regression.
MAPE                     Mean absolute percentage error.

Across the lower columns, for each variable:

Statistic Name           Description
Reg-coef                 Regression coefficients for the variable.
Mexval                   Marginal explanatory value: the percentage increase in SEE if the variable were left out.
Elas                     Elasticity at mean.
NorRes                   The "normalized" sum of squared residuals (SSR) using this and all preceding variables,
                         that is the SSR using this and preceding variables divided by the SSR using all variables.
Mean                     Mean of the variable.
Beta                     What the regression coefficient would be if both dependent and independent variables
                         were in units which made their standard deviations equal to 1.0.

It is always important to ask whether the regression coefficients are of the correct sign and
reasonable magnitude. In this equation, the coefficient on replace, .86737, seems reasonably close
to its theoretical value of 1.0; the coefficients on d are all positive. One might expect them to fade
away to zero more rapidly than they do.

The save file for this regression is meant for reading by the computer, but is legible by humans. In
case you are curious, here is what it looks like for this regression.
       ti Gross Private Domestic Fixed Investment
       f d = gdpR - gdpR[1]
       f replace = .05*@cum(stockf,vfR[4],.05)
       r vfR =   126.662414*intercept +
                 0.867373*replace +
                 0.412527*d[1] +
                 0.444799*d[2] +


                                                                                                                      31
                0.424702*d[3] +
                0.439599*d[4] +
                0.468760*d[5] +
                0.358203*d[6] +
                0.291407*d[7] +
                0.228124*d[8] +
                0.309904*d[9] +
                0.258604*d[10] +
                0.187567*d[11] +
                0.126543*d[12]
     d


As you see, it contains the commands to form the variables and the coefficients of the equation
found by the regression. It omits other data intended to help in thinking about the results of the
regression.

The equation for inventory investment is estimated by executing the following file, called viR.reg:
     # viR.reg - Regression file for inventory investment
     catch viR.cat
     save viR.sav
     lim 1975.1 2007.3
     title viR Change in Inventory
     # fs stands for "final sales"
     f fsR = cR + vfR + xR + gR
     f dfsR = fsR - fsR[1]
     r viR = ! dfsR[1], dfsR[2], dfsR[3],dfsR[4]
     save off
     gname viR
     gr *
     catch off


The ! after the = sign in the r command means to leave out the intercept or constant term. Here are
the results:
                                                            viR Change in Inventory
                            120




                            22




                            -76

                         1975               1980            1985    1990     1995     2000   2005
                                Predicted          Actual




32
     save viR.sav
     lim 1975.1 2007.3
     title viR Change in Inventory
     # fs stands for "final sales"
     f fsR = cR + vfR + xR + gR
     f dfsR = fsR - fsR[1]
     r viR = ! dfsR[1], dfsR[2], dfsR[3],dfsR[4]
     :                             viR Change in Inventory
       SEE   =      25.57 RSQ    = 0.4964 RHO =     0.48 Obser = 131 from 1975.100
       SEE+1 =      22.42 RBSQ = 0.4845 DW =        1.04 DoFree = 127 to    2007.300
       MAPE =      157.01
         Variable name            Reg-Coef Mexval Elas       NorRes    Mean    Beta
       0 viR                    - - - - - - - - - - - - - - - - -      30.88 - - -
       1 dfsR[1]                   0.21434      8.5    0.47    1.35    67.02
       2 dfsR[2]                   0.27478    12.1     0.59    1.00    65.93 0.433
       3 dfsR[3]                  -0.02179      0.1 -0.05      1.00    66.05 -0.034
       4 dfsR[4]                   0.01835      0.1    0.04    1.00    65.87 0.029

     save off
     gname viR
     gr *
     catch off

The fit here leaves room for improvement; but given the extreme volatility and small size of this
variable, this equation should be usable.

Finally, the import equation is estimated by the commands in the mR.reg file
     # mR.reg - Regression file for imports
     catch mR.cat
     save mR.sav
     ti mR Imports
     r mR = xR, vR, cR
     save off
     gname mR
     gr *
     catch off


The results are
                                                                   mR Imports
                           1985




                           1105




                            224

                         1975               1980            1985     1990   1995   2000   2005
                                Predicted          Actual




                                                                                                    33
     save mR.sav
     ti mR Imports
     r mR = xR, vR, cR
     :                                  mR Imports
       SEE   =      68.71 RSQ   = 0.9770 RHO =     0.96 Obser = 131 from 1975.100
       SEE+1 =      22.51 RBSQ = 0.9764 DW =       0.09 DoFree = 127 to    2007.300
       MAPE =        6.41
         Variable name           Reg-Coef Mexval Elas       NorRes    Mean    Beta
       0 mR                    - - - - - - - - - - - - - - - - -     922.12 - - -
       1 intercept             -507.25900   100.6 -0.55      43.44      1.00
       2 xR                       0.20007      2.1    0.16    3.09   732.09 0.126
       3 vR                       0.41272    18.0     0.54    1.62  1217.59 0.318
       4 cR                       0.15560    27.2     0.85    1.00  5015.27 0.557

     save off
     gname mR
     gr *
     catch off


The fit is close enough, but the high coefficient on exports is suspicious. It can hardly mean that 20
percent of the value of exports is imported. Rather, it would seem to be a sort of proxy variable for
the openness of the economy. I am not happy with this equation, but at this introductory stage, let us
accept it.

The master file for Model 5 follows.
     #   Master 5: Regression equations for investment and imports
     #   11 exogenous (not counting time)
     #   6 behavioral ratios for pibg, nitpils, ptax, piipcb, piptt, pisav
     #   3 regression equations for mR, vfR, and viR
     #    xR, gR, npctrR exogenous in constant dollars

     checkdup y
     fex time = time

     # Investment
     add vfR.sav
     add viR.sav

     f vR = vfR+viR
     # Imports
     add mR.sav
     # Gross domestic product
     f gdpR = cR + vR + feR - fiR + gR


     ... and so on exactly as in Model 4.


Note the “add” commands which bring into the model the results of the regressions recorded in the
.sav files.

So, how well does our masterpiece simulate the economy if it is given the historical values of all the
exogenous variables? The graph below shows the answer.


34
                                         gdpR - Real GDP, Simulated and Historical
                                                             Model 5
                          12841




                           7648




                           2455

                         1980             1985        1990        1995   2000        2005
                                b.gdpR       c.gdpR




The model reproduced the general trend of the economy quite well but with a powerful cycle unlike
anything in the real economy. In fact, it is reminiscent of the simple model of section 1 of this
chapter. Thus we see that it is very easy to produce a model which has cycles. All we need is a
multiplier and an investment function based on the “accelerator” theory of investment. Indeed, our
graph should make it fairly clear why our investment equation is called an accelerator; whenever the
economy turns up, it accelerates that upturn. In fact, this feature of Model 5 is so prominent that we
will name Model 5 the Accelerator-Multiplier Interaction model, or AMI for short.

The problem we now face is to find out what stabilizes the economy. We shall return to that question
after two intervening chapters to explain the theory of regression and some more of the mechanics of
model building. You will then be able to estimate the equations, build, and run this model yourself
and go on to work on improving it.




                                                                                                    35
36
             CHAPTER 2. BASIC CONCEPTS OF LEAST SQUARES REGRESSION

In this chapter, we will be concerned with how to sum up the lessons of the past about how one
variable, say the sales of automobiles, depends upon others, such as price and income. We will see
how to fit to historical data equations which express this dependence. Here we will concentrate on
the mechanics of this curve-fitting process and say relatively little about the examination and
interpretation of the results. That will come in due course in later chapters.

      1. What is the Method of Least Squares and Why Use It?

Let us begin directly with a practical problem. Suppose that you need to make forecasts of
automobile sales for the next four quarters. Your first question is naturally, "What factors affect the
sales of automobiles and how strongly does each of them work?"

A good bet for an important factor is certainly disposable income. It also happens to be a convenient
variable because a recent issue of a business magazine shows the forecasts of it made by a dozen
different forecasting firms. Let us presume that you have ten years of quarterly data on both auto
sales and disposable income. How do you find the relation between the two?

The simplest way to answer that question is to plot auto purchases against income as shown on
Figure 2.1 and then draw a line down the middle of the points.

You may use the edge of a transparent ruler or a tightly stretched black string, to help you get a line
which seems to fit. Then, given a forecast of income A, we see that automobile purchases of B
would be right "on line" with that income. Mathematically, what we have done is to determine b1
and b2 of the following equation:

(1)    yt = b1 + b2 xt

where
    yt          =    automobile purchases in quarter t
    xt          =    disposable income in quarter t
    b1, b2      =    constants, often called parameters, to be determined so that the line fits the past
                     experience.

This method of "eyeing in" the line has important advantages over other methods. It allows us to see
what we are doing. It is quick, easy, clear, and makes it easy to spot outliers in the data and to avoid
putting a heavy weight on them. It is, however, seldom used. Theorists don't like it because no
elegant theorems can be proved about it. Practitioners shun it because it is easy to see that it is
subjective; more sophisticated methods allow them to cloak their subjectivity in a more opaque
garment.




                                                                                                       37
The real problem with the method, however, arises when we realize that automobile sales depend on
more than just income. Suppose that we want to take into account the change in income,
dxt = xt - xt-1; then we should have to estimate

(2) y = b1 + b2 x + b3 dx

(The b1 and b2 of (2) may, of course, be different from those of (1).) To estimate these b's by eye in
this equation, we need a three-dimensional construction to suspend the points in space. Then
perhaps we might use a plane of light to find the plane which fits best. But what was simple for one
variable becomes something of an engineering feat for two. But now, of course, we realize that
automobile purchases should also depend on existing stocks, s, of autos, thus:

(3) y = b1 + b2 x + b3 dx + b4 s

Now we need a four-dimensional structure of some sort; and when you have constructed it, be
prepared to tackle a five-dimensional device, for automobile purchases also certainly depend on the
price of automobiles, p, so that we should be estimating:

(4) y = b1 + b2 x + b3 dx + b4 s + b5 p

Here we seem to be pretty much past hope of making a physical picture to aid us in fitting the
equation to the data. If we ever want to estimate an equation with that many parameters -- and for
all practical purposes, with any more than two parameters -- we must find some way to replace
vision by calculation. That switch can be dangerous, and we shall frequently find that we must
check to be sure that the blind calculations have not lost their way.



38
To find a way to calculate the parameters, let us return to the simplest case, equation (1) and Figure
2.1, and let us agree to choose b1 and b2 so as to minimize the sum of the squares of the errors.
Figure 2.2 shows these squares graphically. We twist and push the line through these points until the
sum of the areas of the squares is the smallest. You may ask "Why the squares? Why not just get the
smallest sum of absolute values of error?" Strangely, it is easy to compute the b's to minimize the
sum of squares, but quite difficult to minimize the sum of absolute errors, and it is really this ease
that is responsible for our preference for the squares.




      2. How to Calculate the Least Squares Regression Coefficients

To find the "least squares" values of the b's, we just minimize with respect to b1 and b2 the following
summation:
             T
                                                 2
(5)     S=   ∑    y t−b1 x t1  b 2 x t2  
             t=1
where T is the number of observations which we have, xt1 = 1 for all t and xt2 = income in period t.
The sigma, Σ, of course, indicates summation, and the t=1 below it indicates that the summation
begins with that value of t and extends to t = T, shown above the sigma. Now if S is to be minimal
with respect to both b1 and b2, then it must be minimal with respect to b1 with b2 held constant. The
reader who has not studied calculus must now accept on faith that the solution to equations (8) below
will minimize this S. But anyone who knows that the derivative of x2 is 2x can follow the
derivation. First, the derivative of S with respect to b1 (with b2 held constant) must be zero:
                T
       ∂S
(6)         = ∑ 2  y t −b1 x t1  b 2 x t2   −x t1  = 0
       ∂b1 t = 1
Likewise, S must be minimal with respect to b2, so


                                                                                                     39
                    T
       ∂S
(7)         = ∑ 2  y t −b1 x t1  b2 x t2  −x t2  = 0
       ∂b 2 t = 1
We may see the implications of (6) and (7) more clearly if we will divide through by 2, move the
terms not involving b1 and b2 to the right hand side, and factor out b1 and b2 from the sums involving
them. Upon so doing, (6) and (7) become

       b1 Σ xt1 xt1 + b2 Σ xt1 xt2 = Σ xt1 yt
(8)
       b1 Σ xt2 xt1 + b2 Σ xt2 xt2 = Σ xt2 yt .

(Here we have dropped the limits on the summation simply because they are always the same.) Now
bear clearly in mind that the x's and y's are known, historically observed values. Hence, all the sums
are known numbers. The unknowns are b1 and b2. So we see that we have two linear equations in
two unknowns. You solved equations like that in high school; if perhaps you have forgotten how, I
shall remind you in a moment. Given that we know how to solve linear equations, we can consider
our problem of how to find the b's to be solved.

So far, you may say, we have only dealt with the two-variable case, handled satisfactorily
graphically. What about more variables? Well, we might as well go for the general case and
consider n independent variables x1, ..., xn, and the equation

       yt = b1 xt1 + ... + bn xtn.

Then let
              T
                                                  2
        S = ∑  y t−b1 x t1  ...  bn x tn 
             t =1


Differentiating with respect to b1, b2,...,bn gives
                 T
       ∂S
             = ∑ 2  yt − b1 x t1  ...  b n x tn   −xt1  = 0
       ∂b1      t=1
(9)
                 T
       ∂S
             = ∑ 2  y t − b1 x t1  ...  bn x tn  −x tn  = 0
       ∂b n     t=1


These may be rewritten as

     b1 Σ xt1 xt1 + b2 Σ xt1 xt2 +...+ bn Σ xt1 xtn = Σ xt1 yt
     b1 Σ xt2 xt1 + b2 Σ xt2 xt2 +...+ bn Σ xt2 xtn = Σ xt2 yt
(10) ...................................................
     b1 Σ xtn xt1 + b2 Σ xtn xt2 +...+ bn Σ xtn xtn = Σ xtn yt .

Do you see the system? In the first equation, the first factor in every product is xt1, in the second
equation, xt2, and so on. In the ith column, the second factor in each sum is xti.


40
Let us take a simple example with T = 5.

           t         xt1        xt2        xt3       yt
           1         1          10          5        17
           2         1           5          1        10
(11)       3         1           0          6        12
           4         1          10          3        16
           5         1           0         10        20

You should now verify that the equations (10) are

       5b1 + 25b2 + 25b3 = 75
       25b1 + 225b2 + 85b3 = 380
       25b1 + 85b2 + 171b3 = 415

Table 2.1 shows how to solve them systematically.

           Line       b1      b2      b3 = 1     Derivation

              L1         5.      25.      25.          75.
              L2        25. 225.          85.         380. Original Equations
              L3        25.      85. 171.             415.
         -----------------------------------------------------------
              L4#        1.       5.       5.         15.      L1/5
              L5         0. 100. -40.                 5.       L2 -25*L4
              L6         0. -40.          46.         40.      L3 - 25*L4
         -----------------------------------------------------------
              L7         1.       0.       7.         14.75 L4 - 5*L8
              L8#        0.       1.      -0.4        .05      L5/100
              L9         0.       0.      30.         42.      L6 -(-40)*L8
         ----------------------------------------------------------
             L10         1.       0.       0.         4.95 L7 -7*L12
             L11         0.       1.       0.         .61      L8-(-.4)*L12
             L12#        0.       0.       1.         1.40 L9/30

         A # after a line number marks the line computed first in each panel of three lines.

                                Table 2.1: Least Squares Computations

The first three lines (L1, L2, L3) show the original equations. (The line across the top shows what
the numbers in each column would be multiplied by before adding together to give the equations.)
The next three show the results of eliminating b1 from the second and third equations. To obtain
them, first get the coefficient of b1 in the first equation to be 1.0 by dividing the equation by b1's
coefficient, namely 5. The divisor is called the pivot element and is underlined in the table. The

                                                                                                         41
result is L4. Now to eliminate b1 from equation 2, multiply L4 by b1's coefficient in L2 and subtract
from L2 to get L5. L6 is similarly calculated. We have now completed one pivot operation and
eliminated b1 from all equations except the first. In the next three lines, L7, L8, and L9, we similarly
get 0 for the coefficient on b2 in all but the second equation. Finally, in L10 - L12, we get zero
coefficients on b3 in all but the third equation. From these last three lines, we read the answer

           b1 = 4.95             b2 = .61   b3 = 1.40

The process we have been engaged in is known as linear regression and the b's we have found are
the regression coefficients. The particular method of solution of the equations is known as Gauss-
Jordan reduction. The idea of fitting the equation by minimizing the sum of squared misses first
appears in Adrien Marie Legendre’s Nouvelles méthodes pour la détermination des orbites des
comètes in 1805. In 1809, Carl Friedrich Gauss published a small volume in Latin on the orbits of
planets in which he not only used the method but claimed to have been using it since 1795. Given
Gauss’s many achievements in mathematics, there is no particular reason to doubt his claim, but he
might have forgotten to ever describe it had not Legendre done so first.

                                                  Exercise

2.1 Calculate the regression equation for the following data.

     x1   x2     x3        y
     1    5      2        14
     1    4      3        13
     1    6      3        17
     1    7      5        20
     1    6      7        19
     1    8      6        21



     3. Some Measures of How Well the Equation Fits

Now that we have computed the regression coefficients, we may well ask, How well does the
equation fit the data? To answer, we first need to compute the values of the dependent variable
"predicted" by the equation. These predicted values are denoted by y t thus
                                                                     
                 n
       yt =
               ∑ bi      xt i
               i = 1
They are shown in the third column of Table 2.2, where the actual values are shown in the second
column. The misses, or "residuals",

       r t= y t − y t ,
            

are shown in the fourth column. Note that the sum of the residuals is zero; it will always be zero if


42
there is a constant term in the equation. Since we were trying to minimize the sum of squares of
these residuals, this quantity is naturally of interest:
                T
         S =   ∑1 r t 2
               t=


Actually, the Standard Error of Estimate,

         SEE =        S /T
is easier to interpret for it has the same units as the dependent variable. Indeed, we could describe it
as sort of average error. (Many texts define SEE with T-n instead of T in the denominator. The G7
program uses the above definition as a descriptive statistic in its own right, not an estimate -- under
rather strong assumptions -- of some unobservable population parameter.)
     t          y                      r         r2         d       d2     f       f2        %
     1         17.        18.05       1.05      1.1025     2.0      4.    -1.65    2.7225    6.18
     2         10.         9.40      -0.60      0.3600    -5.0     25.     1.95    3.8025    6.00
     3         12.        13.35       1.35      1.8225    -3.0      9.    -2.10    4.4100   11.25
     4         16.        15.25      -0.75      0.5625     1.0      1.    -0.30    0.0900    4.69
     5         20.        18.95      -1.05      1.1025     5.0     25.     0.00    0.0000    5.25
                                     -----      ------     ---     ---             ------    ----
                                      0.00      4.9500     0.0     64.            11.0250   33.37
          _______
     SEE = √ 4.95/5           =   .995    MAPE = 33.37/5 = 6.67

     R2 = 1 - (4.95/64) = .9227 RBARSQ = 1 -(4.95/2)/(64/4) = .8453

     DW = 11.025/4.95 = 2.2272 RHO = (2 - 2.2272)/2 = -.1136

                                         Table 2.2: The Fit of the Equation


Another measure of closeness of fit is the ratio of S, the sum of squared residuals, to D, the sum of
                                                                    y
the squared deviations from the mean of the dependent variable,  . These deviations are shown in
the column labeled "d" in Table 2.2. This D is, to be explicit,
                 T
         D =    ∑  yt     − t 2
                             y
                t=1
The ratio S/D would be zero for a perfect fit and 1.0 in the worst possible case (provided there is a
constant term in the equation.) Since it seems a bit strange to have zero as the perfect score, the S/D
ratio is subtracted from 1.0 to form what is called the "coefficient of multiple determination" or "R
square" or RSQ for short,

         R2 = 1 − S / D

A "corrected" R2, written with a bar over the R, and pronounced "R bar square" is often used to allow
for the fact that as more and more independent variables are added to a regression, S goes to zero. In
fact, if there are as many independent variables as there are observations, the fit will normally be
perfect. For the corrected R2, therefore, S is divided by T - n, the number of observations less the

                                                                                                      43
number of independent variables. The formula is

                S /T −n
        R2 = 1 −
                 D/T −1

where n is the number of independent variables, counting the constant term. Where it is not
convenient to use superscripts, we call this measure RBARSQ. The number T - n is called the
number of "degrees of freedom" in the regression.

An alternative measure of closeness of fit is the mean absolute percentage error, or MAPE, defined
by
                        T
                               rt

  MAPE =
              100∗ ∑
                       t =1
                              ∣ ∣
                               yt
                         T

Its calculation is illustrated in the last column of Table 2.2.

Although RSQ, RBARSQ, and MAPE are all dimensionless pure numbers, there is no absolute
standard of "how good is good" for them. Generally, any equation whose dependent variable has a
strong trend will have a high RSQ while equations with volatile, untrended dependent variables will
have low RSQ. These measures are really useful only for comparing the fit of one equation for a
particular variable with another equation for the same variable.

The columns of Table 2 labeled f and f2 are explained in the next section.

                                                 Exercise

2.2 For the data in exercise 2.1, calculate also the SEE, RSQ, RBARSQ, and MAPE.


      4. Measures of Autocorrelation and their Uses.

In addition to asking how well the equation fits, it is also usual to ask, To what extent does this
equation tend to "go on making the same mistake"? That is, if it erred on the high side in one period
is it likely to err on the high side in the next period? If so, the residuals are said to be autocorrelated.
The Durbin-Watson statistic, DW, and RHO are both measures of autocorrelation of the residuals
         r t= y t − y t
              

The Durbin-Watson statistic is
                T −1
        DW =     ∑ r t 1 − r t 2 / S
                t=1
and


44
       RHO = 2 − DW /2

is the autocorrelation coefficient,  . RHO is approximately the regression coefficient,  ' , of the
equation
        r t  1 =  ' rt

The formula for this regression coefficient is
              T −1

              ∑ r t1 r t
              t=1
       ' =    T −1

               ∑ rt r t
               t =1




and
       DW =     ∑ r t12       ∑ rt 2    − 2   ∑ r t1 r t
where all of the summations go from 1 to T - 1. For large values of T, the first two summations on
the right and the summation in the denominator of ρ' differ insignificantly from S, so that -- as a
close approximation --

      DW       =      2 - 2ρ'    or      ρ' = (2 - DW)/2.

The computation of DW and RHO for the example is shown in Table 2.2 in the columns labeled f
and f2 and at the bottom of the table. Here ft = rt+1 - rt. The ideal values are 2.0 for DW and 0.0 for
RHO; but, as before, what is a good value of RHO depends on the variables in the equations. For
some variables, it is easy to get RHO under .1, while for others a RHO of .5 may be quite an
accomplishment. Some ailments caused by high values of RHO and some possible treatments for
them are discussed in Chapter 9 in the section on the Hildreth-Lu technique. The definition of the
Durbin-Watson statistic is included here because the statistic is reported by the G7 regression
program. G7 includes it as a concession to the mores of the econometric profession. It is used for
testing the hypothesis that there is no autocorrelation. The tests rest on assumptions that are rarely
valid in building economic models and need not detain us here.

Knowing the value of RHO can be quite helpful when forecasting with an equation. Suppose, for
instance, that we have data through the last quarter of 2006 and want to forecast the first quarter of
2007. We know, moreover, that RHO is .75 and that the equation predicted a value of 326 for 2006
fourth quarter, when the actual value was only 310. The residual was, therefore, 16. Now, for the
first quarter of 2007, the equation is predicting a value of 340. You can accept the prediction at face
value, or you can say to yourself, "I know this equation; if it has predicted too high in one period, it
is likely to predict too high in the next. In fact, the best guess for that error is RHO times the last
previous error. So I am going to assume that it is now missing on the high side by .75*16 = 12, and
my forecast is going to be 340 - 12 = 328. And if I had to forecast the second quarter of 2007, I
would subtract .75*12 = 9 from whatever the equation predicts, and so on into the future."
Forecasts which incorporate such adjustments are called "RHO-adjusted" forecasts. We will denote

                                                                                                          45
the standard error of estimate of such forecasts for one period ahead by "SEE+1".



                                              Exercise

2.3 For the data of exercise 2.1, calculate also DW and RHO. Suppose that you know the
independent variables for two periods further into the future, namely:

          x1 x2 x3
          1 9 7
          1 10 9

Calculate forecasts for these two periods with and without the RHO adjustment.


     5. Statistics for Each Independent Variable

We have now covered the overall characteristics of a regression equation which are normally
displayed by G7, the regression program which goes with this book. You have seen three of these
displays in connection with Model 5 in Chapter 1.

For each individual variable, however, you will see that there are displayed six items labeled: Reg-
Coef, Mexval, Elas, NorRes, Mean, and Beta. The "Reg-coef" is, of course, just the regression
coefficient, the b for each variable, which we have just learned to compute. The "Mean" is just the
mean of the variable over the sample period. The "Elas" is the elasticity calculated at the means.
This elasticity, the percentage by which the dependent variable will change if an independent
variable changes by one percent, is often very useful for examining the economic content of the
regression coefficients. In general, the elasticity of y with respect to x is

     elas = (δy/y)/(δx/x) = (δy/δx)(x/y),

where δ indicates "change in". In a linear equation, such as

     y = a + bx,

(δy/δx) = b, so elas = b(x/y) depends upon the values of x and y at which it is evaluated. It is
customary to evaluate at the means. In the numeric example we worked above, the elasticity of y
with respect to x2 is .61*5/15 = .203. What is the elasticity of y with respect to x3?

The Mexval, or marginal explanatory value, of each variable is simply the percentage by which the
SEE of the equation would increase if the variable were omitted from the equation and the
coefficients of the remaining variables were adjusted to give the best possible fit without that
variable. It helps us to see how important each variable is for the fit of the whole equation. Its
calculation will be explained in section 8 of this chapter.

46
The Beta coefficient shows what the regression coefficient would be if the units of both the
dependent and independent variables were changed so that the variance of each variable was 1.0.
(The variance is the sum of squares of the deviations from the mean divided by the number of
observations.) It is useful for judging the sensitivity of the dependent variable with respect to each
independent variable. For example, in the equation for fixed investment in Chapter 1, the beta
coefficient for the “replace” variable was .781, a coefficient which represents substantial sensitivity.
If “replace” moves by one of its standard deviations, investment will move by about .8 of its
standard deviations.

Many regression programs automatically give Student's t-statistics for each variable. It is intended
for evaluating the “statistical significance" of the dependent variable for the equation. However, the
estimate of the standard deviation involves several highly questionable assumptions, and the
application of the Student t test to the variable requires even further dubious assumptions. For
ordinary, everyday use, the simple, descriptive Mexval is, in my view, far preferable to the T-stat
whose meaning depends on many dubious assumptions. If, however, you wish to see the t-values,
give the command “showt yes”. To return G7 to its normal output display, the command is “showt
no”. Like Mexval, the calculation of T-Stat will be explained in the final section of this chapter.
Activation of the “show t” option in this way also causes F statistics to be printed for each variable.
These F values would be used for testing the combined statistical significance of that variable and all
the following variables in the regression.

The NorRes column shows how the sum of squared residuals was gradually reduced as variables
were added to the regression. The entry on row k shows the sum of squared residuals when the first
k variables had been introduced divided by the corresponding sum when all variables had been
introduced.

                                                Exercise

2.4 . With the data of exercise 2.1, compute the elasticity of the dependent variable with respect to x2
and x3 at the means of all variables.




                                                                                                      47
     6. Matrix Notation for Regression

The calculations described in the previous section can all be expressed very neatly by a shorthand
known as matrix notation. Without it, some of the things to be covered in this book just cannot be
explained. Since it is easy to learn, and will smooth the way for the discussion in the third section of
this chapter, we may as well take a moment to introduce it right here. Perhaps you are already
acquainted with it; if so, just glance at equation (10m) and then skip to the last paragraph of the
section.

To introduce matrices, let us consider again the equations we just solved. They were

     5b1 + 25b2 + 25b3 = 75
     25b1 + 225b2 + 85b3 = 380
     25b1 + 85b2 + 171b3 = 415 .

We could economize on b's and + signs if we would write them instead as



           5 25 25 b 1
                          
                            75
           25 225 85 b 2 = 380
           25 85 171 b 3   415

We have changed nothing by this rewriting; we have just introduced a shorthand. Now let us think
of the array of numbers on the left as a single entity and call it A. This A is a matrix. In fact, any
rectangular array of numbers is a matrix. The column of numbers on the right side is then also a
matrix, but since it has only one column it may also be called a "vector". Because it is a vector, we
will denote it with a lower case letter and call it c. The column of the unknown b's is also a vector,
and we may as well call it b. Then the whole equation can be written as just

     Ab = c.

In this equation, we say that the matrix A is "post multiplied" by the column b. What that means is
fully explained by looking back at the original equations. Here are a couple of examples to check
your understanding of multiplying a matrix by a vector:




                                              
                                           3   1   2         17

           
           3 7
           5 2
           6 1
                   3 =
                 ˙ 2
                       23
                       19
                       20
                                           2
                                           3
                                           4
                                               0
                                               6
                                               1
                                                   8
                                                   4
                                                   0
                                                         2

                                                         5
                                                             44
                                                       ˙ 1 = 32
                                                              9

Note that in order to post multiply the matrix A by the column vector b, b must have the same
number of rows that A has columns. The result will be a column with as many rows as A has rows.
The number of rows and columns of a matrix are called its dimensions. The matrix on the left in the
first example above is said to be "3 by 2", that is it has 3 rows and 2 columns. In general, we write
"A is (m,n)" when we mean that A has m rows and n columns.

48
Now suppose that we have two matrices, A and B. If they are of the same dimensions, we can define
their sum, A + B, as the matrix composed of the sums of the corresponding elements in the two
matrices. For example,


            
          3 5  2 1 = 5 6
          4 3   3 5   7 8

When it comes to the product of two matrices AB, however, we do not define it as the matrix
composed of the products of corresponding elements. Rather we define it as the matrix whose first
column is the product of A post-multiplied by the first column of B, and whose second column is the
product of A post-multiplied by the second column of B, and so on. Here are two examples:


         
          3 5
          4 3
                      2 1   21 28
                    ˙ 3 5 = 17 19    
                         
                          5 2
         3 2 1
          2 5 3          2 1
                                
                                19 13
                        ˙ 1 3 = 21 22    
In order for the product AB to be defined, B must have as many rows as A has columns. The product
will have as many rows as does A and as many columns as does B. In general AB will not be the
same as BA, although there are important special cases in which they are the same.

It is easy to verify that (A + B) + C = A + (B + C) --the order in which we add matrices makes no
difference -- and that (AB)C = A(BC) -- the order in which we multiply makes no difference. Also,
as with ordinary numbers, multiplication is distributive over addition: A(B + C) = AB + AC.

To express our least-squares problem in terms of matrices, we need just one more concept, the
transpose of a matrix. The transpose of A, denoted by A', is the matrix whose first column is the first
row of A, whose second column is the second row of A, and so on. For example, if A is the matrix
given by



                 
           5 2
       A = 1 3
           2 1

then the transpose is


       A' =        5 1 2
                    2 3 1   
If A = A', then the matrix must be square and is said to be symmetric. You can, with a little thought,


                                                                                                    49
see that (AB)' = B'A'.

Now let us use matrix notation to write in compact form the equations of section 2 of this chapter.
We will denote by X the matrix of observations on the independent variables and by y the vector of
observations on the dependent variable in our previous regression example. Thus,




                
            1 10 5                      17
            1 5 1                       10
        X = 1 0 6            ,      y = 12
            1 10 3                      16
            1 0 10                      20

Finally let b denote the vector of regression coefficients. We want to choose b to minimize S, the
sum of squared residuals. In matrix notation, S is

(9m)      S = (y - Xb)' (y - Xb).

This equation has been numbered (9m) because it is the matrix way of writing equation (9) of
section 2. The minimizing b vector is given by the equation

(10m)     (X'X)b = X'y .

It may take a moment to see that this equation (10m) is the same as the previous equation 10. But
just write out X' and X and start forming the product X'X, and you will soon realize that you are
forming the matrix A with which we began this section. Note that X'X is symmetric.

How can we show the process of solution of linear equations in matrix notation? To do so we first
need a special notation for any square matrix that has 1's on the diagonal running from top left to
bottom right and is otherwise zero. Such a matrix is called an "identity" matrix and is therefore
denoted by I. Note that for any matrix M, IM = M and MI = M, where I is an identity matrix of
appropriate dimension. Suppose now that we have the matrix equations Ax = c, where A is a square
matrix and x and c are vectors. If we can find some matrix B such that AB = I, then A(Bc) = (AB)c
= Ic = c, so x = Bc is the solution of the equations Ax = c.

But how do we find the matrix B? Since AB1 = I1, AB2 = I2, and AB3 = I3, where the subscript
denotes a column of B or I, we could just solve these equations one-by-one for the columns of B.
There is, however, an easier way; we can solve them all at once. Table 2.3 shows how. In the first
three rows and first four columns of each panel you will recognize the corresponding panel of Table
2.1. In the fifth column we have written the right-hand side of the equations AB1 = I1.




50
      5.    25.     25.   75.           1.   0.     0.
     25.   225.     85. 380.            0.   1.     0.
     25.    85.    171. 415.            0.   0.     1.
     75.   380.    415. 1189.           0.   0.     0.

      1.     5.      5.     15.      0.2     0.     0.
      0.   100.    -40.      5.     -5.      1.     0.
      0.   -40.     46.     40.     -5.      0.     1.
      0.     5.     40.     64.    -15.      0.     0.

      1.      0.     7.     14.75 0.45 -0.05        0.
      0.      1.    -0.4     0.05 -0.05 0.01        0.
      0.      0.    30.     42.    -7.0   0.4       1.
      0.      0.    42.     63.75 -14.75 -0.05      0.

      1.      0.     0.      4.95 2.083 -0.143 -0.233
      0.      1.     0.      0.61 -0.143 0.015 0.013
      0.      0.     1.      1.4 -0.233 0.013 0.033
      0.      0.     0.      4.95 -4.950 -0.610 -1.400

                            Table 2.3: Regression with Matrix Inversion

Now notice that if we carry the pivot operations through this column as well as the first four we will
have in it in the fourth panel the solution of the equation AB1 = I1. Note that appending this extra
column had absolutely no effect on the previous four columns. Nor did the fourth column, the
original right-hand side of the equations, have any effect on what happened in the fifth column. We
can, therefore, append the other columns of I as the last two columns of the first panel in Table 2.3,
carry through the pivoting on them, and get in the top three rows of the last panel the matrix B. We
have gotten the solution for three sets of equations for considerably less than three times the work
required for one.

The matrix B which we have found in this way is called the "inverse" of A and is denoted by A-1.
From its construction, we know that AA-1 = I. It is also true that A-1A = I, for if we start from the
last panel of Table 2.3 and think of it as representing the equations A-1C = I where A-1 is known and
C is unknown, we can solve these equations for C by retracing our steps one-by-one back up Table
2.3 and discover that C = A. So A-1A = I. Thus, every right inverse is also a left inverse.

We will also often use the fact that the inverse of a symmetric matrix is itself symmetric. If A = A',
then

     I = (AA-1)' = (A-1)'A' = (A-1)'A

and post multiplying both sides of this equation by A-1 gives


                                                                                                     51
       A-1 = (A-1)' ,

which is to say, A-1 is symmetric.

We can now summarize our discussion so far in one equation: the regression coefficients are given
by the equation

(12)        b = (X'X)-1X'y.

For future reference, we need one more concept, the trace of a square matrix. The trace is simply the
sum of the diagonal elements. Thus, if C is (m,m), then the trace of C, tr C, is just
                   m
         tr C =   ∑ c ii
                  t =1


If A is (m,n) and B is (n,m) then AB and BA are both defined but are not generally equal to one
another or even of the same dimension. But both are square so that tr(AB) and tr(BA) are both
defined. Now it is a remarkable and useful theorem that tr(AB) = tr(BA). To see why this is true,
just note that any element of A, say a13, will enter into the sum that forms tr(AB) exactly once and
that one time it will be multiplied by the symmetrically placed element of B, b31. Now notice that
the element also enters tr(BA) exactly once and is again multiplied by the same element of B. Thus
the sums that form tr(AB) and tr(BA) consist of exactly the same elements and must therefore be
equal to one another. Working a small example will make this proof plain.

                                               Exercise

2.5 For the data of exercise 2.1, compute (X'X)-1, then X'y, and then, by matrix multiplication, b =
(X'X)-1X'y.


       7. A Shortcut to the Sum of Squared Residuals

The last row of each panel of Table 2.3 provides a short-cut to the calculation of S, the sum of
squared residuals. We have not yet explained this row. In the top panel, it contains the row vector
(y'X, y'y), which is the transpose of the fourth column, so the first four rows and columns form a
symmetric matrix. As we generate the successive panels, we carry through the pivot operations on
the last row just as on the other rows.

Now let me point out a surprising "coincidence". In the fourth panel of Table 2.3 we find, in the
position where y'y was originally, the number 4.95. This is exactly the value that we got for S by a
totally different procedure in Table 2.2. Can it be that we can find the sum of squares by pivoting
instead of by calculating all of the residuals, squaring them and summing? Yes, that is true. And it is
useful in many ways. Let us see why it is so.

By the time we reach the last panel in Table 2.3, we will have subtracted from the original last row

52
some combination of the rows above it and gotten, in the first three positions, zeroes. What com-
bination of the rows above it did we subtract? Since we originally had in those positions y'X and,
after subtracting we have 0, we must have subtracted a combination, given by the row vector c, such
that c(X'X) = y'X. In fact, this c is really just b', the transpose of the vector of regression
coefficients, for

                 (X'X)b = X'y
so
                 b'(X'X)' = y'X
and
                 b'(X'X) = y'X

since (X'X)' = X'X. Therefore what has been subtracted from the final position of this last row is
b'X'y. What was originally in it was y'y, so what is left is y'y - b'(X'y). The direct approach to
calculating S first calculates

       r = y - Xb = y - X(X'X)-1X'y

and then forms

       S         = r'r
                 = y'y - y'X(X'X)-1X'y - y'X(X'X)-1X'y +
                         y'X(X'X)-1X'X(X'X)-1X'y

                 = y'y - y'X(X'X)-1X'y = y'y - b'X'y,

which is exactly what the pivoting gave.

Now suppose for a moment that we had set out to regress x2 on x1. We would have formed exactly
the same 2-by-2 matrix that we see in the upper left corner of panel 1 of Table 2.3 and the final result
would have been the 2-by-2 in the upper left corner of panel 2. The value of S for this problem
would have been 100 and the regression coefficient would have been 5. Similarly, if x3 had been re-
gressed on x1, the value of S would have been 46 and regression coefficient 5. (Because x1 is the
constant 1, the regression coefficients are the means, and the values of S are the sum of squared
deviations from the means.) In general, we see that in panel i+1, after i pivot operations, we can see
the regression coefficients and values of S for the regression on the first i variables of each of the
remaining variables. Thus, the regression panels show a great deal about the relations among the
variables. Can you give an interpretation for the element in the third row and fourth column of the
third panel (the number is 42)?

For computing the NorRes (Normalized sum of squared Residuals) coefficients shown in the
standard G7 display of results, it is necessary only to store away the value of S, the value in the
lower right corner of the array, at each iteration. These values are then all divided by the last of them
to produce the NorRes display.


                                                                                                      53
Please note that each pivot element was the S value for the variable about to be introduced when
regressed on all the previous values. If a pivot element is zero, the regression cannot continue; but a
zero pivot can occur only if the variable about to be introduced was perfectly explained by the
previous variables. If this happens when, say the third variable is about to be introduced, the G7
program will give the message "Variable 3 is a linear combination of preceding variables," and will
abort that particular regression.

A moment's study of Table 2.3 shows that in each panel three of the columns are the columns of the
identity matrix. In working with a computer, these identity columns just waste space, and it is usual
to store only the non-identity columns of the matrix, as shown below in Table 2.4. I will refer to this
sort of table as regression with compact inversion.


                                               Exercise

2.6     Extend your previous computation with the data of exercise 2.1 to include the "dependent
variable" row in each computation. What is S when only x1 is used as an explanatory variable?
When only x1 and x2 are used? When all three are used? What are the regression coefficients for x3
regressed on x1 and x2?




54
    8. Mexvals and Derivatives -- Measures of the Importance of Each Variable

So far, we have not developed a way to say anything about how important any particular variable is
to the whole equation. One measure designed to help in answering this question is the "mexval" of a
variable. A variable's mexval, or marginal explanatory value, is defined as the percentage that SEE
will increase if the variable is omitted from the regression and not replaced by any other, though the
coefficients on the remaining variables are adjusted to do as well as possible without their departed
comrade.

                         5        25          25           75
                        25       225          85          380
                        25        85         171          415
                        75       380         415         1189

                       0.2         5           5             15
                      -5.0       100         -40              5
                      -5.0       -40          46             40
                     -15.0         5          40             64

                       0.45       -0.05        7.0           14.75
                      -0.05        0.01       -0.4            0.05
                      -7.0         0.4        30.0           42.0
                     -14.75       -0.05       42.0           63.75

                       2.083      -0.143      -0.233          4.95
                      -0.143       0.015       0.013          0.61
                      -0.233       0.013       0.033          1.40
                      -4.950      -0.610      -1.400          4.95

                     Figure 2.4: Regression with Compact Inversion

Mexval is easily calculated in the process of regression with compact inversion. With n independent
variables, this form of regression leads to a final panel like this:

       a11    ...     a1n a1m
       ...   ...     ... ...
       an1    ...     ann anm
       am1     ...     amn amm

where m = n + 1. Remember that the lower right element is the sum of squared residuals. Note also
that the values of the a's do not depend upon the order in which the row reduction was done, that is,
in regression terms, they do not depend on the order of introduction of the variables. Let us suppose
that variable i was the last to be introduced and let us denote the elements of the panel before its
introduction with a'. Then we have the following relation between the a and a':

       aii = 1/a'ii                       and a'ii = 1/aii


                                                                                                   55
       aim = a'im/a'ii                    and a'im = aim/aii
       amm = a'mm - a'ima'im/a'ii and a'mm = amm + aimaim/aii

Thus, the drop in the sum of squares of the residuals as variable i was introduced -- and the increase
in that sum of squares if it is now excluded from the equation -- is aim2/aii.

The standard error of estimate of the equation would therefore rise from

       SEE = sqrt (amm/T)

to

       SEE' = sqrt ((amm + aim2/aii)/T).

where T is the number of observations. Therefore,

       mexvali          = 100*((SEE'/SEE)-1)
                        = 100*(sqrt(1+(a2im/aiiamm))-1).

For the example of the text, we find

       mexval1 = 100*(sqrt(1 + (4.952/(2.083*4.95))) -1) = 83.74
       mexval2 = 100*(sqrt(1 + ( .612/(0.015*4.95))) -1) = 143.0
       mexval3 = 100*(sqrt(1 + (1.402/(0.033*4.95))) -1) = 258.9 .

(The numbers at the right are calculated from a more precise inverse than that shown above.)

The insights of this section and the last can be combined to calculate the derivatives of regression
coefficients with respect to one another. Suppose that we were to decide that we did not trust the
value provided by regression for the coefficient of some variable and that we wanted to fix the value
of that coefficient. What effect would that fixing have on the other coefficients? More precisely,
what would be the derivatives of the other coefficients with respect to that coefficient, if the others
are determined to minimize the sum of squared errors, given the value of the fixed coefficient?
Suppose the original equation was written y = Xb + Zc + r, where Z is a T-by-1 vector and c is a
scalar. We are asking, What is the derivative of the least-squares value of b with respect to c? The
least squares estimate of b, given c, is b = (X'X)-1(X'y - X'Zc), from which it is clear that the
derivative of the vector b with respect to the scalar c is

       db/dc = - (X'X)-1(X'Z),

which is just the negative of the regression coefficients of Z on all the other variables. Now think of
Z as one more independent variable. This negative of the regression coefficients of Z on the columns
of X is just the negative of what would have been in Z's column before it was pivoted on. To get it
back, we just unpivot. In the notation developed in this section, if we want to think of Z as column


56
i, we want a'ji. From the pivot operation, we have aji = 0 - a'ji/a'ii and aii = 1/a'ii. These may be solved
for -a'ji = aji/aii. In other words, to get the derivatives of all of the regression coefficients with
respect to coefficient i, we just divide the ith column of (X'X)-1 by its diagonal element. These
derivatives are very useful for seeing the sensitivity of one coefficient with respect to another. I have
often seen it happen that an equation has several coefficients with nonsensical values, but by fixing
one of the coefficients to a sensible value, the others also became sensible. The derivatives are
useful for spotting such situations.

                                                 Exercise

2.7     Compute the mexvals for x1, x2, and x3 with the data of exercise 2.1. Compute the
derivative of b1 and b3 with respect to b2.


    9. Leverage, a Measure of the Influence of Individual Observations

We mentioned in discussing the fitting of simple regressions by eye that one of the advantages of
that method was that it made clear which observations were outliers -- observations which, if
included, would have a lot of influence on the regression coefficients. Spotting outliers can be
important because the data at the outlier point may be erroneous. One simple indicator of outlier
observations is the "leverage" variable, defined, for observation t, simply as the derivative of the
predicted value at t, y t with respect to the observed value at t, yt. If this derivative is very high for a
                      
particular observation, that observation must have a strong influence on one or more of the
regression coefficients. Such observations should be checked for the accuracy of the data; if the data
is accurate, however, the observation should not be discarded, for it is precisely the observation
which, by standing out from the others, reveals what the equation really is.

The leverage variable is easily calculated. The vector of predicted values is

       y t = Xb = X  X ' X −1 X ' y
       

so the derivative of the predicted value at t, y t , with respect to the observed value at t, yt, is simply
                                                
      th                                       -1
the t diagonal element of the matrix X(X'X) X'. Obviously, if we want only the diagonals of this
matrix, we do not have to compute the whole matrix to find them. Rather, we could pre-multiply the
first column of X' by (X'X)-1 and then pre-multiply this column by the first row of X to get the first
element of the leverage vector.

Note that leverage uses only values of X, not of y, and that observations other than the first entered
the calculation of the leverage on the first observation only through the (X'X)-1 matrix.

The leverage variable is automatically computed and placed in the workspace bank with the name
lever after each regression in G7. It may be graphed or displayed like any other variable.




                                                                                                         57
58
                            CHAPTER 3. INTRODUCTION TO G 7.3

From Chapter 1, you are already somewhat familiar with G7, the regression and modeling program
we will be using throughout this book. It is part of a closely integrated package of programs that can
both estimate a single equation or build a multisectoral model involving hundreds of equations. In
Chapter 1, I tried to tell you as little about G7 as I could; I wanted to concentrate on the logical
succession of models. Here we will proceed more systematically through the basic functions of G7.
Please pardon some occasional repetition of what you already know.

    1. Tutorial of the Basics

Double click on the G7 icon to start the program. You will be immediately given a choice of folder
(formerly called directory) in which to start G7. This step is necessary because G7 looks for a g.cfg
file in the folder from which it starts, and this file determines the characteristics of the workspace
which G creates and the initial assigned data bank. For this tutorial, you should select the c:\ami
folder. To do so, you should click “Browse” on the screen that appears when you start the program
until the c:\ami folder comes into view and then double click the ami folder or (b) single click the
ami folder and tap the ‘Enter’ key, or (c) enter c:\ami\g.cfg in the edit box at the bottom of the
window. Whatever route you choose, you should see “c:\ami” in this box. Then click the OK
button, or tap Alt-O. The folder selection form disappears. Click OK again to proceed.

You then see the G7 main form. It has a menu at the top, a few speed buttons, a white, one-line
command box, and a large blue results area. Click on the Bank menu and select Look. You will see
a list of the assigned banks. Select one of them and click OK. A window opens in the upper right of
the screen. It has a list of the data series in the assigned bank. Run the cursor down the list until you
find one that interests you. Tap ‘Enter’. Below you will see a graph and to the left, in the results
area, the numerical values of the series. (The first number in each line is the date of the first
observation in that line; then follow the data for four successive quarterly observations.) The cursor
will have gone back to the command box, but if you wish to see another series, click on the “look”
window again and continue looking. If you tap a letter key, the cursor will move to the next entry
beginning with that letter. To find a word or phrase, like “Personal income”, click on Find in the
menu and fill in the box. Note that you can search up or down from where you are in the file.

Now let us suppose that you would like to run a regression of real Gross private fixed investment on
past changes in real GDP and an estimate of the investment required to replacement capital that is
wearing out. If the cursor is not already in the command box, click there. We first set the limits for
the regression, the starting and ending dates. To make the regression go from 1975 first quarter to
2007 third quarter, type
     lim 1975.1 2007.3

and tap ‘Enter’. (If your data bank has data more recent than the third quarter of 2007, by all means
use it.) You should see your command echoed in the results area. Then give a title to the regression
with something like
     ti Gross Private Domestic Fixed Investment


                                                                                                      59
Next, you must form d, the first difference of real GDP by
     f   d = gdpR - gdpR[1]

The remaining variable we need is requirements for replacement, which you may calculate by
     f   replace = .05*@cum(stockf,vfR[4],.05)

It is not necessary for the regression, but you may be interested to see how this replacement
compares with investment spending. To draw a graph with the two, type
     gr replace vfR

Now you are ready for the regression itself. Just type
     r vfR = replace, d[1], d[2], d[3], d[4], d[5], d[6], d[7], d[8], d[9], d[10], d[11]


There should now appear in the results area the numeric display similar to what you saw for Model 5
in Chapter 1. The font in the results area is set small so that you see most of the regression output.
Many people, however, find that it is uncomfortably small. If you want a larger font click File |
Auto Font Size, so as to remove the check mark by this item. Then click File | Font and set the size.
Personally, I often pick the font called Fixedsys, which cannot be sized but the one available size
works nicely. You should, however, stick to a monotype font, that is, one in which all letters are the
same width. You may also set the font color and “script” or character set. If you are using titles and
comments in a language such as Greek, Russian, Turkish, or Polish with characters that do not
appear in the Western European languages, be sure to select the proper character set for the results
window. It will be automatically carried over to the editor, the look command, and other displays.
By clicking File | Background color, you can select the background color of the results window. Your
choice of font (typeface, size, color, and character set) and your choice of background color will be
remembered when you next start G7. (This generally pleasant feature can be confusing if you are
using G7 on a public machine in a computer lab; you will start off with the settings of the previous
user.)

To obtain the graph of the regression just done, give the command
     gr *

To see the graph better, it may be maximized by clicking the maximize button, the middle button in
the upper right corner of the graph. Click again the button in the same place to get the graph back to
its original size. If you would like to save the graph for comparison with later graphs, select Graph |
Shift1 from the main menu. The graph shifts to a smaller window in the upper right of the screen.
You can also use Graph | Shift2. Thus, you can view three different graphs at once.

In order to save your work, you should put the commands necessary to do regressions into files and
then execute (or add) those files. To put them in a file, open the G7 editor. Click Editor on the
main menu and a simple editor opens in the upper right screen. Type into it


60
     lim   1975.1 2007.3
     ti    Gross Private Domestic Fixed Investment
     f     d = gdpR - gdpR[1]
     f     replace = .05*@cum(stockf,vfR[4],.05)
     r     vfR = replace, d[1], d[2], d[3], d[4], d[5], d[6], d[7], d[8], d[9], d[10], d[11]
     gr    *


On the Editor’s menu, select File | Save as, and save the file with the name vfR.reg. Then to execute
it, simply click Run in the editor menu. (Clicking Run also saves the file to your disk.) Alternatively,
if the file you want to run is not already in the editor, you can do File | Execute and in the window
which then opens, you may either click the name of the file and then OK, or type in the name and tap
‘Enter’. Either way, you should now see the results and be faced with a “dialog” box demanding to
know what you want done with the graph. For the moment, click the Continue button.

A third way to execute your vfR.reg file is to enter the command
     add vfR.reg


in the white command box. The results are exactly the same.

Your command file, vfR.reg, is fine so far, but it does not save the results in a way which can be put
into a paper or included into a model. Click back on the editor window and modify the file there so
that it is
     addprint y
     catch vfR.cat
     save vfR.sav
     gdates 1975.1 1999.2
     lim 1975.1 1999.2
     ti Gross Private Domestic Fixed Investment
     f d = gdpR - gdpR[1]
     f replace = .05*@cum(stockf,vfR[4],.05)
     r vfR = replace, d[1], d[2],d[3],d[4],d[5],
        d[6],d[7],d[8],d[9],d[10],d[11]
     gr *
     save off
     gname vfR
     gr *
     catch off
The new material is in bold. Save this file and “run” it again. (You will find that the command box
remembers your previous commands. You can get them back either with the down and up arrow keys
or by clicking on the arrow at the right end of the command box.) When the program stops and
demands to know what to do with the graph, click the Save button.

You will have made three new files:

vfR.sav            The results to be used later by the computer in building a model. This file was made by the “save
                   vfR.sav ... save off” pair of commands.
vfR.cat            The results to be brought into a word processor. This file was made by the “catch vfR.cat ... catch
                   off” pair of commands.
vfR.wmf            The graph in a form to be brought into a word processor. This file was made when you clicked



                                                                                                                    61
                  “Save” on the “Continue add file” dialog box.


You can look at vfR.sav with G7’s editor. If the aqua green editor window is showing, click on the
Open item on its menu and open vfR.sav file. Otherwise, first click Editor on the main menu to
create an editor window. You will see that the vfR.sav file contains the commands to create the
variables and the regression results written as an equation.

To appreciate the uses of the other two files, you should open your word processor. You may first
minimize G7 by clicking on the little square in the upper right with the _ in it. Then open your word
processor and import vfR.cat into it. In WordPerfect or Word, you use the command Insert | File. In
OpenOffice Writer, you need to open the file in one window, and then copy and paste it into your
document. When you do so, you are apt to find that the columns look wavy. To correct that
problem, put the imported text into a monotype font such as Courier or Line printer. To do so, select
the text whose font you want to change, click on the button whose caption shows your present font,
and select from the list. If you are using fonts that contain a number of different character sets, such
as Cyrillic, Greek, West European, East European, you should be sure to select also the “script” ( =
character set) you desire.

When you clicked “Save” on the “Continue add file” dialog box, you caused the graph to be saved as
a Windows metafile, a file which has the instructions for redrawing the file on any device supported
by Windows. This file has the name vfR.wmf. It may be imported into a word processing program
such as WordPerfect, Word or OpenOffice. In WordPerfect, the command is the same as to import
text -- Insert | File. Word needs more help from you; the command is Insert | Picture. When it is
first imported, the graph may be small; but it can be stretched by dragging with the mouse. Getting
graphs to go where you want them when you have a number of graphs on a page is a problem with
all present word processors. In WordPerfect, you have a choice of attaching the graph to a page,
paragraph, or character. Probably character is your best choice.

You should now close the word processor and return to G7.

Let us explore the other options on the “Continue add file” dialog box. To do so, give the command
“add vfR.reg” again. (By using the up and down arrow keys you can find this command among
those already executed and just execute it again.) The program will stop, as before, with the
“Continue add file” dialog box demanding to know what to do with the graph. The simplest answer
is to click the “Cont.” button to continue the add file, throwing away the graph. Next to it on the
right is the “Stop” button, which exits from the command file. Further to the right are the “Max”
and “Normal” buttons which blow up the graph to fill the full screen and restore it to its normal size,
respectively. Then comes the “Save” button, which, as we know, saves the graph as a Windows
metafile with the filename specified by a previous gname command. The “Print” button sends the
graph directly to the printer. The “Shift1" and “Shift2" buttons move the graph to storage areas 1
and 2 where the graph remains visible as the add file moves on to subsequent commands. A graph in
these storage areas can also be resized.

In working with multisectoral models, it is common to have add-files which draw graphs for many
sectors. Especially for use with them, “gname” can be started with a name and number, for example:

62
     gname out 1
The name of the save file for the first graph drawn will then be out1.wmf, for the second out2.wmf,
and so on. Note that the numbers increase with each graph drawn whether or not it is saved.

If you have a graph showing and would like to save it but are not in an add file and do not have an
appropriate name already assigned for the file, use Graph | Save from the main menu. You will be
prompted for a file name.

The next step is to use add-files within add-files. Look at the runall.reg with the editor. You will
find
     fdates 1960.1 2007.3
     lim 1975.1 2007.3
     add vfR.reg
     add viR.reg
     add fiR.reg


The “fdates” command sets the range of dates over which the “f” commands work. (The default
fdates are the same as the default regression limits in specified in the g.cfg file.) Adding or
executing this one file will cause three other files to be executed. Try it. After the first graph has
been drawn, you will find that the program stops and demands that you tell it what to do with the
graph. The simplest answer is to click the “Cont.” button to continue the add file, throwing away the
graph.

At this point, your first results on the blue screen will have scrolled out of sight. But you can use the
scroll bars on the blue results window to bring them back into view. You can, by the way, use
standard Windows cut-and-paste techniques to move material from this window to a word processor.
The result may be somewhat disconcerting at first, for the color of the text (yellow) will move with
the text but not the color of the background. If your word processor document has a white
background, you will then have yellow text on a white background. In WordPerfect, the reveal
codes key allows you to find the color change command and delete it, revealing the text in the
normal text color. In Word or OpenOffice, commands in the Format menu control these colors.

As you work, the results window may become so full that tiny movements of the scroll bar produce
big changes in what text is shown. You can clear the results window at any time by clicking File |
Clear results or by giving the “clear” command in the command box or in an “add” file. The results
area is large but not infinite. If you come close to filling it up, the oldest part at the top is
automatically deleted.

You will notice also the item File | Auto font size. By clicking it, you can toggle a check mark next
to it. If the check mark is visible, then widening or narrowing the results window by dragging its
right border with the mouse will cause the size of the font to change so that the basic regression
display fills the screen. If you do not want this automatic change in the size of the font, click this
item so that the check mark disappears. This feature has no effect if using the FixedSys font, which
gives type of pleasant size and clarity.

All of the commands under the Edit item on the main menu apply to the results window.


                                                                                                       63
The Help menu explains the commands available from the command line in G. See especially the
help reference section.


     2. Digression on Graphs in G

While it is very simple to get a quick graph in G7, it is also possible to refine the graph. In order to
produce nice looking reports, it is worth digressing from model building for a moment to learn what
sorts of graphs you can make.

The dates over which series are graphed is controlled by the “gdates” command. This command
may be followed by two or three dates, for example:
     gdates 1980.1 2007.3
     gdates 1980.1 2007.3 2015.4


Graphs drawn following the first of these commands will cover the period 1980.1 — 2007.3.
Graphs drawn following the second cover the same period but have a vertical line following 2007.3.
This vertical line is useful, for example, to divide history from forecast or the period over which a
regression was fit from the period over which it was tested. Exception: graphs drawn with the “gr *”
command use dates set by the “limits” command for the previous regression.

For greater variety in your graphs, choose Graph | Settings on the main menu. You will get a
window for setting the characteristics of up to seven lines on the graphs. You can select the color,
width (in pixels), style (solid, dotted, dashed, dash dot, and dash dot dot -- these work only for lines
1 pixel wide), mark, fill, and left and right parameters. The last three apply only if the type “bar” is
chosen for the mark. The fill has to do with what sort of hatching appears in bars. The left and right
parameters, numbers between 0 and 1, show where, within the space allocated for the bar, the left
and right edges of the bar go. For example, with left = .1 and right = .9, the bar will be in the center
of the allowed space, and the bars will be four times as wide (.8 units) as the space between them (.2
units). To eliminate the line connecting points marked by bars, set its width to zero. Experiment
with what you can do with these options. If you wish to save your graph settings for a future run of
G, click the “Save to file” button at the bottom of the screen. You will be asked for a filename. Any
valid file name is acceptable, but let us suppose you say “graphs.set”. Then the next time you start G
you can give the command “add graphs.set” and have your settings back. You can even add at the
bottom of the g.cfg file the line
     Initial command; add graphs.set

and you will automatically have back your settings.

Normally, G7 automatically adjusts the vertical scale to include all points on any series being
graphed, but only the top, bottom and middle of the vertical scale are labeled. You can, however,
control the vertical range of the graph with the “vrange” or “vr” command. For example:



64
     vr 0 500 1000 1500 2000

will cause graphs to be drawn with a range from 0 to 2000 and horizontal lines at 0, 500, 1000, 1500,
and 2000. These lines will be labeled. The default is to put the labels inside the box of the graph, but
you can put them outside by first giving the vertical axis label command, “vaxl”. It can be
     vaxl out

to put them outside the frame of the graph or
     vaxl in

to put them inside the frame. The “vr” command continues in force until another vr command is
given. To simply turn it off and go back to the default, use
     vr off

One particularly useful form of the “vr” command is
     vr   0

which sets the lower boundary of the following graphs to be 0, while the top is picked by the
program to fit in the highest point in any series. Try graphing gdpR with careful vertical axis
labeling done with the “vr” and “vaxl” commands.

In addition to the “title” command, which we have used frequently, there is the “subtitle” command,
which can be abbreviated to “subti”, which provides a subtitle for the graph. The title and subtitle
remain until replaced by another title or subtitle. To remove either of them altogether, just give the
command immediately followed by a carriage return. The “legend” command controls whether or
not the legend normally at the bottom of the graph is included or not. The format is “legend yes” or
“legend no”. Try using the “subtitle” and “legend” commands.

In working with multisectoral models, it is common to have add files which draw graphs for many
sectors. Especially for use with them, “gname” can the started with a name and number, thus
     gname out 1

The name of the save file for the first graph drawn will then be out1.wmf, for the second out2.wmf,
and so on. Note that the numbers increase with each graph drawn whether or not it is saved.

Besides the ordinary “gr” command, there are three other variants of it. The “mgr” or “multi graph”
command chooses a separate scale for each series graphed. For example, you may want to put the
Treasury bill rate, rtb, and Residential construction, vfrR, on the same graph. But they have totally
different scales. The answer is to make the graph with “mgr”, for example, “mgr rtb vfrR”. Try it.

Semi-logarithmic graphs are popular because series growing at a constant percent per year appear as
straight lines. It is, however, often desirable to label the vertical axis in the original units. This is
done by the “lgr” command. For example, to graph the Standard and Poor’s composite index of 500

                                                                                                       65
stocks from 1980.1 to 1999.2, we could do
     f lsp500 = @log(sp500)
     vr 100 200 400 800 1600 3200 4800
     gdates 1980.1 2007.3
     lgr lsp500


Do it and note the straightness of the line and the labeling of the vertical axis in the original units.

Finally, for graphs in which the horizontal axis is not time but some other variable, use the “sgr”
or “scatter graph” command. For details, see the help file.


     3. Building a Model

From Chapter 1, we are already familiar with building and running models that do not include
regression equations. The last model, Model 5, involved regression, but we did not build or run it.
Here we add the inclusion of regression equations and review all the steps.

Four of the six steps in building a model appear under the Model menu item. The first step is to
create a master file which specifies the regression result files — the .sav files — which will be used
in the model and other identities necessary for the model. Selecting Model | Master will show the
current contents of the master file in the editor. Note that each regression equation is brought into
the model with a command of the form “add xxx.sav” , where xxx is the usually the name of the
dependent variable in the regression. This must be the same name that appeared on the “save” line
in the .reg file by which the equation was estimated. When you are adding an equation to the model,
the first thing you should do is to modify the master file to include the new equation. Generally, it is
not crucially important where the “add” command appears, but it is natural to place it after the
determination of the explanatory (right-hand-side) variables which it uses.

At the bottom of the master file there should be one or more “check” commands. Model 5, for
example, has the command
     check cR    .2

The check commands control the iterative solution of simultaneous models as explained relative to
Model 5 in Chapter 1.

Note carefully that you conclude the editing of a Master file by clicking Save on the editor menu —
not by clicking Run. You want to use the Master file as input into the Build program, not the G
program. Clicking Run will accomplish nothing useful and may consume a good bit of time as you
respond to various “bad command” messages.

The second step is to estimate all the equations and save the results in .sav files. This step is not
mentioned under the Model menu items, because it is involves all the other capabilities of G. We
have already seen several examples and the following chapters have many more.


66
The third step is to build the model. This step is accomplished by selecting Model | Build. For it to
work completely, the C compiler must have been installed. This step runs the Build program, which
reads the master file and all the .sav files, writes a C++ program, hearta.cpp, to execute the model,
and compiles and links this program to produce an executable file, run.exe, which is the model in the
form of a program ready to run. This step also creates

bws                     A data bank containing all the variables used in the model.
run.gr                  A command file for G to graph all the variables in the model.
run.err                 A record of all the diagnostic information provided by the Build program. Be sure to check it by
                        looking at it with the editor. If you have included the command “addtype y” in your Master file,
                        this file will also contain everything in the Master file and in the files which are “added.” If you
                        include at the top of the Master file the command “checkdup y”, Build will check every “f” or
                        “fex” command to see if it is calculating a variable which has been calculated before and, if so,
                        whether the values are identical. This check is a powerful way to spot a source of frequent errors
                        in building models. Check your run.err file often.
exogall.reg             A command file for G7 to create mechanical projections of all exogenous variables in the model
                        by regressing them on time. Look at exogall.reg to see what it does. The projections will be put
                        into files with the variable name as the name of the file and “xog” as the extension.
run.xog                 A file to use all the .xog files made by running exogall.reg.
skipall.fix             A file to “skip” the calculation of all the variables calculated by regression equations. When all
                        the regression equations are skipped, these variables become essentially exogenous and the
                        model becomes tautological. Hence, its historical simulation should completely reproduce
                        history. If it fails to do so, there is something wrong with the identities. Since it is very easy to
                        make mistakes in the identities, it is always important to run a model with all regressions
                        skipped. This file makes it easy to do so.
run.nam                 A file with the names of all variables in the model and their numbers necessary to read the
                        hearta.c program should you have to do so. With luck, you won’t need to ever look at heart.c or
                        run.nam.
heart.dat, heart.dim,   These files are internal to the system. You should not need to ever look at them.
run.lag, run.map


The fourth step is to run the model. It is accomplished by selecting Model | Run. You will be asked
to fill in a form with the starting and stopping dates, the name of the output bank, and the name of
the “fix” file. For the quarterly demonstration model, use the most recent twenty years for which you
have data. The first simulation can be a historical one with actual values for all of the exogenous
variables. Hence we may name the output bank “histsim”. The “fix” files are where we specify
various alternative scenarios or assumptions for forecasts. For the moment, we want a fix file with
only rho adjustment values. This file is called rhoadj.fix, but we leave off the .fix in entering its
name in the dialog box. When you click the OK button, you get a ] prompt. Give a full title to your
run, something like
      ti Historical Simulation of Model 4

You get the ] prompt again, and you type “run”, and the model runs.



                                                                                                                            67
The fifth step is to look at the results graphically. To do so for a historical simulation, we need to
assign in one bank the actual data in the bws bank and in another the alternative for comparison —
here histsim. We do so with the commands
     bank bws c
     bank histsim b

Then we have a number of “title”, “gname”, and “graph” commands to make the graphs. We can
put all of these in an add file, such as hist.shw, when we execute the file by
     add hist.shw


A simple hist.shw file is may look like this:
     bank histsim b
     bank bws c
     gdates 1980.1 2007.3
     title GDPR -- REAL GROSS DOMESTIC PRODUCT
     gname gdpR
     gr b.gdpR c.gdpR

     title cR -- CONSUMPTION
     gname gcR
     gr b.cR c.cR

     title vfR -- GROSS PRIVATE FIXED INVESTMENT
     gname vfR
     gr b.vfR c.vfR


and so on for as many series as we want to graph. In the “gr” lines, the “b.” and “c.” tell G7 to look
for these series in and only in bank b or bank c, respectively. The effect of this command, therefore,
is to graph the gdpR series from histsim as the first line, the red line, and then to graph the gdpR
series from bws as the second, the blue line. The result is a graph with the actual historical course of
gdpR shown in blue (true is blue) and the model’s calculated gdpR shown in red.

The sixth step is to make tables. Table-making is found on the Model | Tables menu item. Selecting
this item brings up a form for preparing input data to the Compare program, which will actually
make the tables. At the top of the form, you are asked for the name of the “stub” file. The stub file
fundamentally gives the names of the series to be put into the table and the dates to be listed. It may
contain, however, many commands to the Compare program which are described in the G7 help
files. An appropriate initial stub for the AMI model is in the file ami.stb, so put “ami” in the stub file
edit window. We will come back to the contents of the stub file in a moment.

At the top right of the form is space for the name of the “output file,” the text file which will be
produced as the output of the comparison. An acceptable entry would be “histsim.out”.

Next, you must specify how you want to show the alternative bank. The base bank, the first, will
always be shown in actual values. The others, however, may be shown as deviations from the base,
as percentage deviations from the base, or as actual values. Usually — but not always — the best
choice is deviations, so that is the default value.

68
Then we must enter the types and names of the banks we want to compare. The types are easy:
throughout Parts 1 and 2 of this book, the type is always workspace. Since we are making a
comparison of a the histsim bank against the bws bank, put “bws” in the name field for the first bank
and “histsim” in the name field for the second bank. Then click OK.

Compare is a console application program; it will run in a window. Normally, there is nothing to do
except tap any key when the program has finished. However, if Compare finds fault with your
commands, it will let you know in this window, so look for messages before blithely tapping a key.

When Compare has finished and the window has closed, look at the output file, histsim.out, in our
example, with the editor. Be sure that the editor is using a monospaced font, such as Courier, Line
Printer, or Fixedsys; otherwise, the columns of numbers may not line up vertically.

Now let us look at that ami.stb file. Here it is.
       \dates 1990 1995 2000 2001 2002 2003 2004 2005 2006 2007
       \under =
       \7 1 65 2 2 45
       ;
       &
       gdpR         ;Real Gross domestic product
       cR           ; Personal consumption
       vR           ; Gross private domestic investment
       vfR          ;    Fixed investment
       viR          ;    Inventory change
       xR           ; Exports
       mR           ; Imports
       gR           ; Government purchaes
       ;
       gdpD         ;GDP Deflator
       gdp          ;Gross domestic product
       pibg         ; Personal Income before gov
       nitpils      ; - Taxes on production & imports
       ptax         ; - Personal taxes
       npctr        ; + Gov transfers to persons
       pidis        ; = Personal disposable income
       piipcb       ; - Interest paid by consumers
       piptt        ; - Transfers to foreigners
       c            ; - Personal consumption expenditure
       pisav        ; = Personal saving

The commands beginning with a \ are commands to Compare. The first one, and the one you may
well wish to change, sets the dates for the data to be shown. A date like 2000 is an annual date, and
Compare will take an annual average of the quarterly data in our model. We could also give a
quarterly date like 2000.2. If we include intervals in the dates, like 2000-2005, Compare will show
the growth rate over that period. The growth rates will be calculated with continuous compounding.
(Other compounding specifications are available, but are generally inferior to the default.) The
\under command just sets the character which will be used for underling in the table.
The \ command by itself, as in the third line, sets the following parameters:

       fw      field width of the numbers in the table

                                                                                                      69
       dp      decimal places to be shown in the table
       pl      page length, in lines
       tm      top margin, in lines
       bm      bottom margin, in lines
       tw      width of the titles on the left side of the page.

An & at the beginning of a line causes the dates to be written across the page and underlined with
the underline character. A line with a * in the first position forces a new page. The other lines, that
is to say, most lines, have variable or expression on the left followed by a “ ; “ followed by a title.
The title will appear at the left of the page and the value of the expression will be shown in the
columns under the appropriate dates.

There are many other Compare commands. You may also wish to look at Compare.pdf which is in
the pdg directory.

You are now in a position to begin trying to use the software by doing the exercise at the end of the
chapter.


     4. Using Help

As is usual in Windows programs, the last item on the menu is Help. When you click on it, you
should get a window with two books, one labeled Tutorial and the other labeled Command reference.
If you click on either of them, it will open and show you the topics in it. To go to one of these
topics, double click on it or put the cursor on it and tap the ‘Enter’ key. The text for the topic will
appear in a box at the right of the screen. You may need to enlarge the window to read it easily. The
tutorial is similar to what you have just read in this chapter. You can move forward or backward
through it by using the “browse” buttons at the top, marked by >> and <<. After the tutorial, you
will find the command reference for G7. These commands are those which you give in the white
command box or include in a command file to be “added.” If you click on “contents” at the top, you
get back to the window with the books. If you click “index” you get references to keywords which
the help author identified in the text. When you pick one of these, you get in the lower window a list
of topics where the keyword occurred. If you still do not find what you want, you can do a full text
search of the help material by clicking on the “Find” tab and waiting — at least the first time —
while the computer makes a listing of every word that occurs at any point in the help text.

In the Command reference section, you should first read the first three topics. After that, each topic
should be self-contained. It is my own experience in using help files for Windows programs that
they are often clipped, circular, and unclear. They try to substitute hypertext links for old-fashioned
clarity. In contrast, I hope that you will find the G7 help text clear and with what you need to know
all in one place. Examples are plentiful.




70
                                               Exercises

3.1.   Create and run .reg files for all of the regressions used in Model 5 of Chapter 1. Modify the
       Master file as shown in Chapter 1 to make the model include these regression equations.
       Build and run the model in historical simulation. Make graphs and tables. In doing this
       exercise, you should read over the section on Model 5 in Chapter 1 and then try to do the
       whole exercise without looking again at Chapter 1. Only if you get hopelessly stuck should
       you look back before finishing. Then you might want to compare the way you did the
       exercise with the way I did it.

3.2.   Develop an equation relating employment (emp) to real GDP, gdpR, include it in your model,
       build and run the model in historical simulation. You should:
               Modify the master file to use the new equation
               Create a file with the editor to estimate the equation
               Run that file so as to get the results in a .sav and a .cat file.
               Choose Model | Build from the main menu
               Choose Model | Run to run the new model
               Edit your .shw file to put in a graph for employment
               Execute this file and save the employment graph
               Edit ami.stb to put in a line for employment
               Choose Model | Tables to make the numerical listing of the results.

       When all the computing work has been done, go into your word processor and bring in your
       new master file, your emp.reg file, the resulting .cat file, the graph of the fit of the equation,
       the graph of employment in the historical simulation of the model, and the table comparing
       the historical simulation with actual values. You will find that to show the text files correctly
       with numbers in nice straight columns, you need to use a monotype font such as Courier or
       Line Printer.




                                                                                                      71
72
                       CHAPTER 4. FORECASTS AND EXPERIMENTS

Let us take stock of where we are, what we have done, and where we have to go. In Chapter 1, we
saw what a model was, but we learned little about how to actually make one. In Chapter 2, we
looked carefully at theoretical aspects of fitting equations to data. In Chapter 3, we first saw how
this curve fitting was done in practice with the G7 program and how the equations can be combined
into a model, and how that model can be run in historical simulation, that is, with historical values of
all the exogenous variables. In this chapter, we will see how to run counter-historical simulations or
“experiments with the past” by changing the values of one or another of the model’s exogenous
variables from what they actually were to what they might have been. The model will then spell out
the effects of these changes on all the endogenous variables in the model. In this way, it gives us a
sort of controlled experiment by means of the model. We will also see how to use the model to make
forecasts. We have but to forecast the exogenous variables and then use the model to calculate the
endogenous variables. Indeed, we can make not just one forecast but a number of them, with
different projections of the exogenous variables. Usually a number of the exogenous variables
represent policy variables such as government spending, tax rates, or money supply. Thus,
comparison of the outcomes of the simulations with different projections for these variables enables
us to evaluate the wisdom of pursuing various policies. This use of models is perhaps their most
important application.

Unfortunately, I have found no way to prevent users of the G7 software from occasionally making
egregious errors that cause their models to explode or collapse or, more rarely, hang. There are,
however, several tools for finding these errors, and they are described at the end of this chapter in the
section on debugging of models.

Some of the deficiencies of AMI are already visible and more will become apparent in this chapter.
In the next chapter, Chapter 5, we will look at some techniques for developing equations which can,
to some extent, remove those deficiencies. That chapter ends with a list of problems which outline a
research program for turning AMI into a fully respectable macroeconomic model. You can, at that
point, set to work on making your own model. The final chapter, Chapter 6, reviews a number of
qualitative factors in judging an equation. Fundamentally, an equation should make sense. It
should, at a minimum, be conceivable that such a relation as you are estimating should exist in the
economy over a number of years. It is surprisingly easy to write down equations that fit fairly well
but make no sense. Chapter 6 develops a checklist of pitfalls to look out for.

Your work through the rest of Part 1 will be an iterative application of the material in the last four
chapters. To improve the simulation or forecasting properties of your model, as found using the
techniques of this chapter, you will look for new relations using, perhaps, the techniques described in
Chapter 5. Then you pass your new equation in review before the checklist of Chapter 6. When it
passes, you return to the techniques of Chapter 3 to include it in the model, and build and run the
model in historical simulation. Then you come back to the methods of this chapter to test the model
in counter-historical simulation and forecasting. When you are satisfied with those results, you are
ready to tell the world -- or at least your class -- about your model.




                                                                                                      73
     1. Counter-historical Simulation

You might well suppose that a good performance in historical simulation would be a quite adequate
test of a model. Indeed, it is a necessary condition for a model to be useful. But it is by no means
sufficient. For it is easy to build a model that does quite well in a historical simulation, especially
over the period for which it was fit. The trick is simply to make some key variables exogenous or
derived by identity from exogenous variables.

For example, the equation for fixed investment in AMI, (in the vfR.reg file) contains the lines
     f   d = gdpR – gdpR[1]

Now if we change from defining d in terms of gdpR to defining it in terms of gnp$, thus
     f   d = gnp$ - gnp$[1]

we will see no appreciable change in the fit of the equation but a big improvement in the historical
simulation of the model. Try it for yourself. It works like magic, stage magic, that is, for the
improvement is only in the appearances, not in the essence. Indeed, the essence of the model is
much damaged by the change. Why does the change work such wonders on the historical
simulation? Just because gnp$ is not otherwise in the model. Formally, it is exogenous although
logically it should be endogenous. But because it is exogenous, when the historical simulation is
run, fixed investment will be calculated from actual, historical real GNP, not that calculated by the
model itself. Thus, the simulated value of vfR will be almost exactly the predicted value found in
fitting the equation. That fit, you will recall, was much closer than that of the historical simulation
where the simulated value, not the historical value of gdpR, had to be used in calculating d.

We will call this sort of deceptive exogeneity false exogeneity. It usually occurs by accident,
unthinkingly. But the possibility of this occurrence is enough to make us leery of judging a model
by its historical simulation alone. And so we are led to try some counter historical simulations.

A counter-historical simulation is just a simulation over the historical period with one (or possibly
more) of the exogenous variables set at values different from what they actually were historically.
The simplest way to do this is to start the run of the model (with Model | Run) just as you would for
a historical simulation except that in the bottom box where you have previously given “histsim” you
give a name (of eight or less letters and no spaces) for the data bank to be created by this run. For
example, if you are going to increase government purchases of goods and services by $20 billion,
you might call the bank “gRup20". When you click OK and get the ] prompt, give a title something
like
     ti Counter-historical simulation with gR increased by $20 billion.
You then get the ] prompt again. Give the command
     type gR
and you will get the historical values of gR displayed on the screen and the ] prompt again. This
time give the command
     update gR +20
and then again
     type gR


74
By comparing what you now get with what you got from the first ‟type gR”, you should see that
each and every value of gR has been increased by 20. You can abbreviate ‟update” to ‟up” . To
subtract $20 billion, the command would be
       up gR -20
or to increase each value by 20 percent,
       up gR *1.20
To specify the complete course of gR to see, for example, the effects of a major reduction in gR
beginning in 1975 and falling from 875 at the beginning of the year down to 800 by 1977.2 and
remaining there for the year and then rising steadily to 1000 by 1997.2, you can use the following:
       up gR
       1985.1   875   870 865 860   850 840 830         820
       1987.1   810   800 800 800   800 0 0 0
       1989.1   0 0   0 0   0 0 0 0   0 0 0 0           0 0 0 0       0 0 0 0       0 0 0 0
       1995.1   0 0   0 0   0 0 0 0   0 0 0 0           0 0 0 0       0 0 0 0
       2000.1   0 0   0 0   0 0 0 0   0 0 0 0           0 0 0 0       0 0 0 0
       2005.1   0 0   0 0   0 0 0 0   0 1000.


Here we have made use of the capacity of the program to replace a zero by a linearly interpolated
value. In effect, the program draws a straight line from the 800 in 1988.1 to the 1000 in 2007.2.
Remember that the first number on each line is the date of the first observation on the line. There is
no effective limit on how many numbers can be on the line. As you can see, you have the capacity to
run a wide variety of counter historical simulations by altering the exogenous variables.

You can also modify the values produced by the equations of the model. You could ask, for
example, how would the economy have behaved if investment had always been 20 percent above
what the equation predicted. Such modifications of the predicted values are referred to as ‟fixes”
There are four types of fixes available to you:

cta             Constant term adjustment (also called "adds"). Add a specified amount to the value calculated by the
                equation. The amount added can be different in different periods.
mul             Multiplicative fix. Multiply the value calculated from the equation by a specified factor, which may be
                different in different periods.
ovr             Over-ride the equation. Discard the value calculated and use the one specified.
rho             Add a rho-adjustment to the variable
skip            Ignores the value predicted by the equation and uses the historical value in its place.


The format for the first three is similar:

       fixtype <variable_name>
       data lines

For example
       cta cpcR
       1998.3 -20 -20       -25 -30 -30 -25         -15 -12 -10 -8        -4    -2 ;




                                                                                                                       75
The format for the rho-adjustment fix is

     rho <variable_name> <rho_value>

for example
     rho vfR .9


The format for skip is just

     skip <variable_name>

for example
     skip cpcR


These fixes work only when applied to variables which are the dependent variable in a regression.
When applied to other variables, the forecasting program does not reject them; it just never uses
them. Furthermore, only one can be applied to any one variable. If more than one is applied, only
the last takes effect. For example, if somewhere in the fix file after the above "cta cpcR" there
should appear "rho cpcR .9", the cta will be completely forgotten by the program without any
warning.

Constant term adjustments are useful but easily abused. If, for example, construction of a major new
oil pipeline had recently been announced, it might be fair to assume that the equation in the model
doesn't know about the announcement and would forecast better with the help of a constant term
adjustment. On the other hand, the worst abuse is to piece out the imperfections of the model in
running a scenario. For example, if the scenario is an increase in money supply, and the model fails
to show an increase in investment, it is an abuse of the cta to use one to pump up investment and to
claim that the rise was a result of the increase in money supply, something "we know has to happen
even though the model refuses to show it." This abuse, unfortunately, is not unknown in the
commercial model building industry. In my own forecasting, I make extensive use of rho
adjustments, which are a sort of automatic cta, but I seldom use a cta directly.

The values of rho in the various regressions can be conveniently captured for use in a .fix file by
G7’s “rhosave” command. The format is
     rhosave <filename>
for example,
     rhosave     rho.fix

After giving this command, estimate all the equations by doing “add runall.reg” — a good reason for
keeping your runall.reg file up to date as you add more regressions to the model. Then give the
command
     rhosave off



76
and the file in which the values of rho are being saved will be closed and available for importing into
the rhoadj.fix file or any other .fix file.

When making tables from a counter-historical run, the base of comparison is nearly always the
historical simulation and never the actual historical values. Why? If we compare with the historical
simulation, we know that the differences we are looking at are the result of the changes we made in
the exogenous variables or in the equations by the fixes. Whereas if we compared the counter-
historical simulation with the actual historical values, the differences would be a combination of
errors in model and the effects of the changes we made in the exogenous variables. The whole value
of having the model is to isolate the effects of the changes you made, so be sure to use histsim as the
base against which to compare your counter-historical simulation.

                                                Exercise
4.1     Do the suggested counter historical simulation with gR increased by $50. Then try $100 and
        $200. (The table maker will let you have up to four alternatives.) Does there appear to be
        any limit on how big gdpR can be made by increasing government expenditure? Is anything
        bad happening as a result of increasing gR? Do you believe that what you have found to be
        properties of the AMI model are in fact properties of the U.S. economy? If not, you have
        used counter-historical simulation to reveal defects of a model. How do you think AMI might
        be improved?


      2. Forecasting

A forecast is made, of course, by running the model over a future period. Like any other run, it is
made by clicking Model | Run and filling in the blanks. But, unlike historical and counter historical
simulations, a forecast will not have sensible values of exogenous variables unless you provide them.
You do so via the .fix file.

The work of forecasting therefore centers on preparing this fix file. It must contain projections of all
the exogenous variables and may contain also instructions for modifying the results of the regression
equations by cta, mul, ovr, and rho fixes.

I suggest that you make a file called fore.fix to hold this information. To make it, start G7's editor
(File | Editor) with a blank screen. (If the editor already has something in it, use the Editor's File |
New command to clear the screen.) Then use the Editor's File | Insert command to insert the file
rhoadj.fix. The top line of the file gives a title which you should be sure to change for your forecast.
With the cursor just below this line -- that is, at the beginning of the second line -- select File |
Insert, and insert the file run.xog. This file was created when you did Model | Build. Here is what it
contains for the basic AMI model.
      add   time.xog
      add   gdpD.xog
      add   xR.xog
      add   gR.xog



                                                                                                      77
     add   pibgBR.xog
     add   nitpilsBR.xog
     add   ptaxBR.xog
     add   npctrR.xog
     add   piipcbBR.xog
     add   pipttBR.xog
     add   pisavBR.xog


As you see, it is a series of ‟add” commands to include in the data for your forecast the values of
each exogenous variable in the model. You must create the .xog files with the future values of each
exogenous variable. (In the AMI model, the first is for ‟time”; future values for it are already in the
model's historical bank, and you should delete the line for it in the fore.fix file.) You should now
save this file. You are perfectly free to save it with any legal file name provided that you remember
what it is, but I will assume that you used fore.fix.

Now you must get to work creating the .xog files that have the future values of the exogenous
variables. The format of these files is familiar. Future values are specified by "update" commands
exactly like the update (or up) commands used in the counter historical simulation but with future
dates. (Of course, update commands like ‟update xxx +20" will not work because there is nothing to
which to add the 20.) When forecasting, you can use linear interpolation to automatically fill in zero
values in the exogenous variables. (Actual values of zero for an exogenous variable should be
replaced by very small values.)

For exports, feR, we could express its forecast in either of two possible ways in the feR.xog file. We
could have
     update feR
     1999.100      1000.224     1022.952     1044.259     1064.444
     2000.100      1083.743     1102.343     1120.391     1138.003
     2001.100      1155.271     1172.267     1189.048     1205.661
     2002.100      1222.139     1238.512     1254.802     1271.026
     2003.100      1287.198     1303.329     1319.428     1335.501
     2004.100      1351.554     1367.591     1383.615     1399.629
     2005.100      1415.636     1431.636     1447.632     1463.624


or
     update feR
     1999.100      1000.224   1022.952   1044.259    1064.444
     2000.100      0 0 0 0   0 0 0 0   0 0 0 0    0 0 0 0 0 0 0 0
     2005.100      0 0 0 1463.624



The second makes use of the interpolation abilities of the update command, whereas the first
provides a value for every period. Because this projection is very close to a straight line, there is
little practical difference between the two.

The first of these updates was prepared in a very mechanical way by regressing feR on time and
using the predicted value of the forecast using the rho adjustment. The historical values followed by
this forecast are in the variable called depvar created by G7 when the "mode f" has been given. (The


78
f in "mode f" is for "forecast".) The G7 commands to create this file were
       bank bws b
       mode f
       tdates 1999.1 2005.4
       limits 1992.1 2007.3 2010.4
       ti xR
       r b.xR = time
       gr *
       f xR = depvar
       save xR.xog
       sty xR
       save off


This command file first assigns the data bank “bws”. This bank is the one created when the model
was built. It has all the variables in the model and no others. Most of the variables in the model are
in the Quip data bank which is automatically assigned when G7 is started, but some, such as taxrate
and the various behavioral ratios, are made up in the process of building the model. By assigning the
bws bank as bank b by
       bank bws b
these variables become available by putting a “b.” in front of their name. In this command file, the
limits command caused the equation to be fit from 1992.1 to 2007.3 and then forecasted to 2010.4.
The forecast worked because the only independent variable is time, one of only two variables with
future values in the Quip bank. After the forecast is completed, the depvar variable (depvar =
dependent variable) contains the actual values of the dependent variable for the historical period and
the forecasted values with rho adjustment for the future period. The command
       f   xR = depvar
gives xR not only the correct historical values but also the forecast. After opening the xR.xog file for
saving, the command
       sty xR
causes the past and forecasted values of xR to be written to xR.xog. (sty is short for “silent type” —
“silent” because the values are not written to the screen.) This command wrote into the xR.xog file
the first of the "update xR" displays shown above. In the regression, we could have used, besides
       r xR = time
also
       r xR = xR[1]
or even
       r xR = time, xR[1], xR[2]
for lagged values of the dependent variable are available for forecasting. The resulting forecasts are,
of course, somewhat different.

The fore.fix file may contain either the command ‟add xR.xog” or the contents of the xR.xog file can
be brought into fore.fix and the line "add xR.xog" deleted. I myself generally prefer this second way
of working, because it puts all of the projections in one place where they are easily seen by the user.

The sort of mechanical projection illustrated here for exports makes sense for a few variables like
population and possibly exports. It does not make sense to use it for tax rates, defense expenditures,
and other policy variables. Projections of such variables should be thought about carefully in the
light not only of their recent trends but also of current political developments. Mechanical forecasts
can also be made for these variables, but only as a starting point for thinking about them.

                                                                                                     79
If you discover any surprise exogenous variables in run.xog, ask where they came from. Did you
really mean for them to be exogenous or are they just sort of accidental exogenous variables? If the
latter, you have false exogeneity and should modify the model to eliminate it. If you find yourself
making exogenous projections for variables that are clearly not independent of the endogenous
variables in the model, revise the model to eliminate them. Typical examples are to find that gdp
(current dollar GDP) is exogenous while gdpR is endogenous. Or that the consumer price index
(cpi) is exogenous while the gdp deflator, gdpD, is endogenous. If you find yourself making
exogenous projections of variables that are not largely external to the economy in the short run (such
as population and labor force) or that do not represent policy (such as money supply, tax rates, or
government spending), stop! Your model has in it one of those tricks that both make the historical
simulation look good and render the model useless. Get rid of the false exogeneity by either adding
an equation for the variable or use a variable that is already endogenous in place of the offender.

After this exhortation to think about your projections of the exogenous variables, I must now confess
that the Build program automatically made a file for making these mechanical projections of all of
the exogenous variables. It is called exogall.reg. To use it, you must first edit the second and third
lines to specify the dates over which you want the regression run and the projections printed. (If your
forecasts fail to go as far into the future as you intend, you probably need to add an “fdates”
command to specify the range over which “f” commands work, for example:
     fdates 1960.1 2020.4


That done, just execute the file, and you will get .xog files for all the exogenous variables. Look at
one or two of them such as xR.reg or gR.reg. The fore.fix file can then be made to use all these
exogenous projections inserting into it (File | Insert) the run.xog file.

I repeat, use these mechanical projections only as starting points for thinking about economically
justifiable projections. When you have thought about a particular exogenous projection and edited
the .xog file that contains it, I suggest that you save your edited version with the extension .xg
instead of .xog. Edit fore.fix to use the .xg file. The reason for this suggestion is to avoid accidental
destruction of your carefully edited files by doing “add exogall.reg” again, perhaps because changes
in the model have added some new exogenous variables.

Inclusion of the rho adjustment factors is particularly important for a good short-term forecast, for
they avoid abrupt changes in variables as the model "jumps onto" the equation. Use the rhosave
command described above to get all the rho fixes together in a convenient way.

You can, of course, use the cta, mul, and ovr fix types as well. If you have used a "cta", "mul", or
"ovr" adjustment on any variable, say x, you can check the values of that adjustment by doing "ty
x:f" at the ] prompt.

Experiments with the model can be very useful both in testing it and, once you are satisfied with it,
in learning about the economy.



80
                                                Exercise

4.2      Make a forecast with AMI over the next five years. You will have to make independent
        forecasts of all exogenous variables. Put them, as described above into a file that you may
        call "fore.fix". When your "fore.fix" file is ready, make the forecast with something like the
        following on the form obtained by Model | Run
                Start date     2007.4
                Stop date      2012.4.4
                Fix file       fore
                Output bank my4cast
        You should, of course replace the dates by dates appropriate to the time at which you are
        working. The Start date must be a period for which full data are available. The name
        "my4cast" is just an example. Be sure that the fore.fix file has in it rho fixes for all the
        regression equations.

        Use G7 to graph the forecasts. You will probably want to create a "fore.shw" file which will
        make graphs with nice titles. Make tables showing your forecast. To do so, you will need to
        edit the \dates line at the top of the ami.stb file to show the periods you are interested in. To
        get growth rates, put the dates between which you want the growth rate with a hyphen
        between them in place of a date in this line. You may find it convenient to keep a copy of the
        stub file with these dates as fore.stb.

4.3.    Make other forecasts with AMI with different values of the exogenous variables. Compare
the results both graphically (with the gr command) and numerically (with Model | Tables).


      3. Debugging Models

You may be so lucky as to never have a model that behaves so badly that you know it must have a
serious error in it. Chances are, however, that sooner than you would like you will encounter a
serious error. The model may not converge, it may produce values of NAN (Not A Number) for
some variable, may take the logarithm of a negative number, divide by zero, produce utterly
ridiculous values for one or more series, or otherwise complain that it is sick and needs your care.
The G7 software gives you various diagnostic tools for finding the problem.

Even if the model shows no evident problem, you should check the correctness of its identities by
running a historical simulation and, at the ] prompt, give the command
      add   skipall.fix
This skipall.fix is a file created by Build which applies a “skip” command to every regression
equation. Consequently, this historical simulation should precisely reproduce the data, that is the
bank it creates should be identical to the bws bank. To find out if they are identical, assign one as
bank b, the other as bank c, and add run.gr, thus
      bank histsim    b
      bank bws c
      add run.gr
All the graphs should show the two lines on top of one another so that they look like one line. If you

                                                                                                        81
see two lines on any graph, you know you have an error in the identities.

The most common problem is that you have given two different variables the same name. This error
is virtually certain to cause trouble if lagged values of either variable are used. G7 has a special
check for this problem. Be sure that the master file begins with the command ‟checkdup y”; this
command will cause Build to check for duplicate names. If it finds such a problem, it will record it
in the file run.err. If you also have “addprint y” somewhere before the spot where the trouble occurs,
you can pinpoint exactly which statement is changing the value. You should look at the run.err file
with the G7 editor and search for the word “Error”. If you find such a change of definition, you
should correct it.

Another debugging tool is the use of the “maxit” command when running the model. You will recall
that in each period the AMI model goes through a cycle of computations to go from the assumed
value of disposable income to the implied value, and then repeats this cycle with the implied value of
the first round as the assumed value of the second round. An error in the model will normally make
the implied value from the first round nonsense and then everything in the second round nonsense.
To spot where the error is, it can be helpful to limit the number of iterations to one. Then the
variables calculated in the first period are sensible down to the point where the error occurs; but
from that point on, one or more of them is nonsense. Here are the steps in this process. We shall
suppose that the historical simulation which is giving trouble starts in 1980.1.

1. Run a historical simulation from 1980.1 to, say, 1985.4, but when you get the ] prompt where you
would normally type ‟run”, type first
     maxit 1
and then run. The program will then perform one iteration, report that it has not converged, and ask
what you want to do. Answer g for go to next period. Let us say that the bank you produced in
this way was called histsim.

2 In G7, do
     bank bws c
     bank histsim b
     gdates 1978.1 1980.1 1985.4
     add run.gr


The red and the blue lines should be identical before (to the left of) the vertical line and should be
close to one another to the right of it. If you find a difference before the vertical line, there is an
error in an identity or “f” statement. If there is a big difference to the right (but no difference to the
left), there is an error in the regression equation. With this sort of pinpointing of the error, you
should be able to spot it.

If neither of these methods shows up the error, take the model back to its simplest form by removing
all the regression equations. To do so, just put a # in from of each line of the form ‟add xxx.sav” in
the master file. If there are no errors in the identities, that should certainly produce a model which
runs. Then put the equations back in one by one (by removing the #'s), each time running the model.
When the equation you have just put back causes the model to malfunction, you know that the
trouble is in the interaction between that .sav file and what is already in the model. That process


82
should certainly identify which .sav file is making trouble and enable you to see the problem.

If you already know how to use the Borland C++ debugger and wish to use that knowledge to debug
your model, you need to know that the model itself is in the file hearta.cpp and run.nam gives the
correspondence between the names of the variables and their position in the vp array in the program.
It is beyond the scope of this book, however, to explain the use of the debugger. I very seldom find
it necessary or helpful for these models.

Exercise 4.4. Check the identities of the AMI model.




                                                                                                  83
84
                    CHAPTER 5. TECHNIQUES FOR IMPROVING A MODEL

The exercise at the end of the previous chapter pointed to a major problem with AMI. As you
increased government spending, real GDP and just about everything good got better and better. It
would seem that all problems could be solved by the government’s just buying more goods and
services. Many of the early econometric models from the 1950's and 1960's had this sort of
simplistic Keynesian character. Today almost no one would argue that they were adequate
representations of economic reality. Making a model show the adverse effects of liberal fiscal (and
monetary) policy will take us through some of the major problems of macroeconomics of the last
thirty years. It will also require us to introduce some new tools for working with regression.

What happens if the government tries to achieve very high levels of employment by increasing
spending? Any student of elementary macroeconomics knows, or believes he knows, that inflation
will accelerate, perhaps in a never ending spiral. (If your macro is a bit rusty, look up in your macro
book the Phillips curve with the acceleration modification.) According to this theory, it is not the
absolute level of inflation which depends on unemployment, but the deviation of inflation from its
expected value. This expected value, in turn, is usually thought to be a weighted average of past
values of inflation. While the theory says nothing about the distribution of weights in the weighted
average, a common assumption is that the weights decline exponentially. Such a variable can be
easily created with G7’s @cum() function.


    1. Working with the @cum Function

The “cum” in @cum is short for “cumulate”. The function creates a variable, y, from a given
variable, x, by the following formula:

      y t=1−s  y t −1 x t

where s is a number between 0 and 1. In G7, we would create y from x by this equation with the
formula
     f y = @cum(y,x,s) .


(The @cum function is slightly strange in that the series which is the result of the cumulation --
which I shall call the cumulate and is here indicated by y -- must be given to the function as an
argument. That is because, when the model is running, the @cum function must know the previous
value of y as well as the current value of x in order to calculate the current value of y.) We can write
the formula for yt as a function of the preceding values of x as
           ∞
      y t=∑ 1−s x t−
          =1


For example, if s = .1, then by the time we get to t = 100, we would have


                                                                                                      85
      y 100 =1.0x100 .9 x 99 .81 x 98.72 x97.656 x 96...

The weights form a geometric series with sum equal 1/s if we take infinitely many terms -- and very,
very close to 1/s if we stop when the individual terms become less than, say, .01 . In many
applications, such as the present one, we would like to have the sum of weights equal to 1.0. We can
achieve that in either of two, not quite equivalent, ways. The first is to multiply s, since that is
equivalent to dividing by 1/s. For example, if s is .1, we can define y by
     f y = 0.1*@cum(cx, x, 0.1)


This y will have the same dimensions as x. Note that we had to introduce a different variable, cx,
which is the pure, unmultiplied cumulate of x as the first argument in the @cum function.

The @cum() command has an approximate physical analogy in the filling of a leaky bucket from a
water pipe. The rate of inflow is x, the level of the water is y; the outflow through the leak is
proportional to the level of water in the bucket. Thus the outflow equals sy and s is called the spill
rate.

The problem with this way of making the weights sum to 1.0 is that it does not work exactly,
especially not near the beginning of the data bank when we have only a few observations. The
second way solves this problem by dividing by exactly the sum of weights of the terms so far
included. To get this sum, we just compute the cumulate of the series which is 1.0 from the
beginning of the data bank. Following the analogy mentioned above to a bucket with a leak in it,
these cumulates of 1.0 are called “unit buckets.” Here is the formula for calculating the ten-percent
unit bucket, which we may as well call ub10:
     f   ub10 = @cum(ub10, 1.0, 0.10)           .


Let us now apply these ideas to computing expected inflation.


     2. Rates of Change with Continuous Compounding

The measure of the price level which we shall use is the GDP deflator, for that is the one we shall
need to compute GDP in current prices, which we shall soon need. This variable appears in the Quip
data bank with the name gdpD. It measures the level of prices in any quarter relative to the average
in the base year, currently 2000. It is thus not directly a measure of inflation. Rather, the
corresponding measure of inflation is the percentage rate of change of gdpD. How do we compute
this percentage rate of change? One possibility would be
     f infl = 400.*(gdpD - gdpD[1])/gdpD[1]


Why the 400 on the right instead of the 100 you perhaps expected? Because we are working with
quarterly data so the change observed is only over a quarter of a year, but inflation — like interest
rates with which they are often compared — are conventionally stated in percent per year, not per

86
quarter. Please compute this variable and graph it. You will find a very jagged series. To get a
smoother series which we have a better chance of explaining, we will take inflation over the last
year. It can be calculated in G7 by
     f infl = 100.*(gdpD - gdpD[4])/gdpD[4]

An alternative definition is to take the natural logarithm of gdpD and the to take the difference of the
logarithm, like this:
     f   lgdpD = 100.*@log(gdpD)
     f   infl = lgdpD - lgdpD[4]


The first definition corresponds to the idea of annual compounding of growth rates, while the
logarithmic definition corresponds to continuous compounding. For natural growth processes,
continuous compounding is the only natural way to measure the rate of growth. All others introduce
an extraneous element — the period of compounding — into describing the process. Further,
continuous compounding is the only way to get the same growth rates both up and down. If, for
example, a series goes from 100 in year 1 to 120 in year 2 and back to 100 in year 3, annual
compounding says it went up by 20 percent and then fell by 16.67 percent to get back to where it had
been. Thus the rises seems to have been bigger than the fall! The continuous, logarithmic definition
says it rose by 18.23 percent in the first year and fell by 18.23 percent in the second, so that the two
changes are recognized as being of the same magnitude but in opposite directions.


    3. Inflation I — The Wolf in Sheep's Clothing

Now with inflation defined, we can undertake to define expected inflation with the @cum function.
We would have
     f   lgdpD = 100.*@log(gdpD)
     f   infl = lgdpD - lgdpD[4]
     f   ub10 = @cum(ub10, 1.0, .10)
     # call expected inflation “inflex”
     f   inflex = @cum(cinfl, infl[1], .10)/ub10

Put these commands into G7 and graph the series you have created. Since the data bank begins in
1960, you might want to begin some of the graphs back near that date to watch how the @cum
function works.

Before we can try an equation for inflation, we need to define unemployment in terms of the
employment variable for which you already have an equation. Besides emp, the Quip data bank also
contains a variable for the civilian labor force, lfc. To compute the unemployment percentage we
need
     f   u = 100.*(lfc - emp)/ lfc


You can now put all the preceding ideas together and formulate a regression as follows:


                                                                                                     87
     r infl = inflex, u, u[1], u[2], u[3]


The fit, shown below, over the period 1977.1 to 2007.3 is surprisingly good considering that only the
unemployment rate has been used as an explanatory variable.


                                                   Inflation I
                       9.77




                       5.40




                       1.04

                              1980          1985   1990      1995   2000      2005
                         Predicted      Actual




Now put the equation in the model and run it. Because we are interested here in testing how well
this equation simulates when it has to use (in the definition of inflex) the prior values of infl which it
itself generated, I have isolated this equation from the rest of the model. Namely, I have used actual
unemployment, not that generated by the model. This change required simply replacing the line
defining u, given previously by
     f     u = 100.*(lfc      -      emp)/ lfc
by
     fex     u = 100.*(lfc        -    emp)/ lfc


You will recall that a “fex” is used for forming exogenous variables. It puts the variable on the left
into the model, but not the equation. When we go to actually use the model, we certainly want u to
be endogenous. For the moment, however, so that we can concentrate on the interaction of the
inflation equation with itself, the “fex” definition is desirable. Subsequent simulations of the
inflation equation in this chapter are all done with the equation isolated in this way.

Once the model has been run in historical simulation, plot infl from the bws and histsim banks. The
result, shown below, is not bad from 1982 to 1994, but in 1997, the equation begins to explode. The
fit is those years after 1997 was not great, but it certainly did not explode. The forecast continues to
diverge from the actual throughout the rest of the forecast period.




88
                                                                     infl - Inflation
                          11.7




                           6.4




                           1.0

                                         1985               1990                   1995             2000     2005
                            b.infl                 c.infl




The beginning of that explosion warns us that we should look carefully at the equation for explosive
tendencies. One way to check for them is to run the model with constant values of unemployment.
Since we have left unemployment exogenous, we can easily set its values when we run the model.
The lines marked by + signs in the graph below shows the results of running it from 1982.1 to
2001.4 with three different constant values of unemployment, namely 4, 5 and 6 percent, to produce
the top, middle, and bottom lines, respectively. Here, the dangerously explosive nature of this
equation becomes apparent. It is hard to believe that at an unemployment rate of 4 percent, prices
would go into an upward spiral reaching 301 percent per year after 25 years! Indeed, this equation
turns out to be a wolf ready to eat alive an unsuspecting modeler.
                                                                     infl -- Inflation
                                                                               Equation1
                           301




                           151




                            1

                         1980                   1985               1990               1995            2000    2005
                                a.infl             b.infl             c.infl               d.infl




This example shows the value of a historical simulation, for while we have seen that a good
historical simulation is no guarantee of a good model, a bad one, such as we have here, is surely an

                                                                                                                     89
indication of an unusable model.

Since the equation is misbehaving badly, we had best have a close look at it. Its numerical output is
shown below.
                                          Inflation I
          SEE    =      0.68 RSQ   = 0.8974 RHO =     0.90 Obser = 123 from 1977.100
          SEE+1 =       0.30 RBSQ = 0.8931 DW =       0.20 DoFree = 117 to    2007.300
          MAPE =       21.05
            Variable name           Reg-Coef Mexval Elas       NorRes    Mean    Beta
          0 infl                  - - - - - - - - - - - - - - - - -        3.53 - - -
          1 intercept                3.75940    44.1     1.07    9.75      1.00
          2 inflex                   1.36107   175.6     1.48    2.42      3.84 1.280
          3 u                       -0.35922      0.6 -0.62      1.29      6.13 -0.238
          4 u[1]                    -0.25623      0.1 -0.45      1.11      6.15 -0.170
          5 u[2]                     0.56479      0.5    0.99    1.07      6.18 0.374
          6 u[3]                    -0.83326      3.3 -1.47      1.00      6.20 -0.550


Look at the coefficient on inflex: 1.36. That means that if expected inflation rises by one percentage
point, inflation itself will rise by 1.4 percentage points! Notice in the graph of the fit that there were
positive errors in 1997 for several consecutive quarters. When these quarters were hit in the
simulation, expected inflation rose well above actual inflation and the 1.4 “perpetuation factor” took
hold and generated this runaway inflation. It could very well happen, by the way, that a few
consecutive quarters of misses on the low side could start a plunge of inflation down into negative
values.

A more reasonable value for the coefficient of inflex would be 1.0 or perhaps something a bit below
1.0. Values above 1 for this coefficient are almost certain to make for an unstable model. Of course,
G7 did not realize that our inflex was expected inflation. It had no idea of what was a reasonable
value for its coefficient. It just picked one to minimize the sum of squared errors. How can we tell
G7 what would be a reasonable value? That question brings us to the matter of soft constraints.


     4. Soft Constraints

Suppose that we want to suggest to G7 that a value for b2 of .9 would be appropriate. How can we
do so? We can simply manufacture some new, artificial observations. In these observations, the
value of x2, inflex in the present case, is 1.0, the value of infl, the dependent variable, is .9, and the
values of other variables are 0 (including x1, the variable which is multiplied by the intercept). Thus,
we have an observation that looks something like this

     x1     x2   x3 ... xn    y
     0      1    0 ... 0     .9

The residual from this observation will be .9 - 1b2. Pretty clearly the value of b2 which minimizes the
square of this one residual is .9. Of course, there are other residuals whose squares go into the sum
of squared residuals which is being minimized, so including one such observation won't result in a b2
which is exactly .9 . But as more and more such observations are added, the value of b2 will certainly
move towards .9. Thus, we can say that by the addition of such observations we are softly

90
constraining the estimated regression coefficients.

The command in G7 for adding one such artificial observation for each natural observation is just
     con 1 .9 = a2


and to add 10 such observations per natural observation is
     con 10 .9 = a2

The general form of the constraint or con command is
    con <count> <left> = <right>
where
    count     is the number of artificial observations per natural observation.
    left      is a number, the left side of the equation which expresses the constraint.
    right     is any linear expression in the coefficients of the equation.

Some examples of con commands illustrating the ‟right” element are
     con 3 .9 = a2
     con 200 0 = a2 - 2a3 + a4
     con 1000 1 = a3 + a4 + a5 + a6          .


(The letter a is used here to denote the regression coefficients instead of the b which we have used
in the text because G7 is able to estimate several regression equations at once with constraints across
the equations. In such tasks, a is used for the coefficients of the first equation, b for coefficients
of the second, and so on. We will come to such problems in Part 2.)

The natural question at this point is: “How do I know how many of the artificial observations to
use?” Is it really conceivable that, as the third example seems to imply, I would ever want to use
1000 artificial observations per natural observation? The only general rule about how many to use is
‟As many as it takes to accomplish what you want done.” And yes, it is quite common to take large
numbers of artificial observations, because their impact depends on the dimensions of the variables.
Suppose, as in the third example above, that we are trying to constraint the sum of four coefficients,
all of them between 0 and 1, to be 1.0. The discrepancy is likely to be .2 or .4 at worst. Now if the
dimension of the dependent variable is such that the SEE for the equation is 1200, then it is going to
take a lot of observations with an error of .2 to make the least-squares algorithm ‟listen” to the
constraint. Indeed, it is going to take a thousand, or maybe ten thousand. Experiment to find out
what it takes for your particular equation.




                                                                                                    91
     5. Inflation II — A Rough-hewn Timber

Let us return now to estimating the equation for inflation and add to the previous commands just this
one:
     con 20 .9 = a2


The resulting equation is shown here.
     :                                       Inflation 2
            SEE    =      0.87 RSQ   = 0.8311 RHO =     0.93 Obser = 123 from 1977.100
            SEE+1 =       0.32 RBSQ = 0.8239 DW =       0.14 DoFree = 117 to    2007.300
            MAPE =       22.28
              Variable name           Reg-Coef Mexval Elas       NorRes    Mean    Beta
            0 infl                  - - - - - - - - - - - - - - - - -        3.53 - - -
            1 intercept                2.44062    15.3     0.69   26.36      1.00
            2 inflex                   0.93505   401.0     1.02    1.42      3.84 0.880
            3 u                        0.46945      0.6    0.82    1.14      6.13 0.310
            4 u[1]                    -0.73701      0.5 -1.29      1.02      6.15 -0.488
            5 u[2]                     0.30117      0.1    0.53    1.01      6.18 0.199
            6 u[3]                    -0.43614      0.5 -0.77      1.00      6.20 -0.288



The constraint did the job required of it and got b2 down just barely under 1.0. The fit is worse, as
you can see from the increase in SEE or by comparing the graph on the left below with the earlier
graph of the fit. But the simulation results, as you can see on the right, are improved. We have an
equation which we could conceivably use, but it lacks finesse, as we shall see.
                              Inflation 2                                                         infl - Inflation 2
                              Regression Fit                                                      Historical Simulation
     9.77                                                       6.71




     5.40                                                       3.87




     1.04                                                       1.04

            1980       1985   1990         1995   2000   2005              1985            1990            1995           2000   2005
       Predicted   Actual                                         b.infl          c.infl




How does this equation hold up under testing with constant values for unemployment? The graph
below shows runs with constant unemployment at 4, 5, and 6 percent. These results are certainly
more plausible than the results with the first version of the equation, though still not acceptable.




92
                                                     infl -- Inflation 2
                                                                 U = 4, 5, 6
                           10.44




                           5.74




                           1.04

                         1980            1985        1990                1995       2000   2005
                                a.infl      b.infl      c.infl             d.infl




It is often difficult to get newcomers to model building to apply soft constraints because they tend to
have some sort of feeling that “the computer knows best” or that it isn’t “scientific” to provide
information and understanding that is not in the database. The considerable improvement in the
simulation in this case should provide some assurance that thinking about equations and imposing
requirements based on those thoughts can make our models more capable of imitating economic
reality.


    6. Inflation III — Accounting for External Shocks

If we look closely at the fit of this last inflation equation, we see that the biggest problems arise in
the 1976 to 1980 period, the time of the second “oil shock.” At the time, journalists and “the man in
the street” were blaming the whole of inflation on the oil shock. We have seen that we can explain
the broad outlines of inflation without mention of oil prices, but it is at least worth an experiment to
see if we “imported” the high inflation of those years by a faster rise in the price of imported goods
than in the price of comparable domestic goods. For “comparable” domestic goods, we will use U.S.
exports. While we obviously do not export the same goods that we import, both aggregates involve
tradable goods and services. The measure of the relative prices we will use is calculated by the line
     fex   relpri = 100.*@log((m/m$)/(x/x$))

and its rate of inflation, which we can call imflimp for “inflation imported” is
     f inflimp = relpri - relpri[4]


We have used the official deflated values of imports and exports, because their deflators reflect the
specific prices of the goods traded, whereas our xR and mR are both deflated by the same deflator.
We will define relpri by a “fex” rather than by an “f” because we want to think of this variable as
exogenous. A littler experimentation with the lags led to the equation
     r infl = inflex, u, u[1], u[2], u[3], inflimp[2], inflimp[3]



                                                                                                        93
The graph of the fit, on the left, looks very good; there are only two or three periods of noticeable
errors. The inflimp variable helped explain the drop in the inflation rate in 1997, and the rise in
inflation in 2001. Though on the low side, the historical simulation on the right shows a further
improvement in the last five years.

                              Inflation 3                                                    infl - Inflation 3
                              Regression Fit                                                  Historical Simulation
     9.77                                                         9.77




     5.37                                                         5.40




     0.97                                                         1.04

            1980       1985   1990         1995   2000   2005   1980            1985        1990          1995        2000   2005
       Predicted   Actual                                              b.infl      c.infl



     :                                       Inflation 3
            SEE    =      0.64 RSQ   = 0.9104 RHO =     0.84 Obser = 123 from 1977.100
            SEE+1 =       0.35 RBSQ = 0.9049 DW =       0.32 DoFree = 115 to    2007.300
            MAPE =       15.82
              Variable name           Reg-Coef Mexval Elas       NorRes    Mean    Beta
            0 infl                  - - - - - - - - - - - - - - - - -        3.53 - - -
            1 intercept                1.85385    16.1     0.53   50.45      1.00
            2 inflex                   0.91802   568.7     1.00    2.73      3.84 0.864
            3 u                        0.58866      1.9    1.02    2.19      6.13 0.389
            4 u[1]                    -0.85569      1.2 -1.49      1.95      6.15 -0.566
            5 u[2]                     0.04208      0.0    0.07    1.93      6.18 0.028
            6 u[3]                    -0.08868      0.0 -0.16      1.91      6.20 -0.059
            7 inflimp[2]               0.04700      1.0    0.01    1.17      0.53 0.079
            8 inflimp[3]               0.13552      7.9    0.02    1.00      0.53 0.227


There is, however, an anomaly in the numerical results of all of the equations we have used so far.
We turn now to correcting it.


     7. Inflation IV: Using Distributed Lags

Look at the coefficients for the u variables in the last regression. The first is positive, the opposite
one would expect; the second is negative; the third is positive again; and the fourth is again
negative. That is certainly not what I had in mind when I put in several lagged values of
unemployment. I just thought it might take some time for prices to respond to unemployment. I had
no expectation that the direction of causality should reverse in the middle and must confess that I
would be very uncomfortable forecasting or simulating the economy with such an equation. As with
the coefficient on inflex, I expect that the plausibility of the results of our model would be much
enhanced by smoothing out those three coefficients. By “smoothing out” I mean making them lie

94
more nearly on a straight line. If they lay exactly on a straight line then we would have

     a3 - a4 = a4 - a5

or

     0 = a3 - 2a4 + a5

If a6 is also on that line, then

     0 = a4 - 2a5 + a6.

And if that line passes through zero one period further out, then

     a6 = .5a5
or

     0 = a5 - 2a6


We can impose these constraints with the “con” command previously introduced, thus

     con 2 0 = a3 - 2a4 + a5
     con 2 0 = a4 - 2a5 + a6
     con 2 0 = a5 - 2a6

This use of the “con” command in this way to impose a smooth pattern on coefficients is quite
common. But it frequently happens that we have a number of lagged values of some variable,
perhaps five or ten, and we want to impose smoothness over all their estimated coefficients. For
example, in Chapter 1, the first regression we looked at (with vfR as dependent variable) had 12
lagged values of d, with coefficients a3 through a14. If you look closely at these coefficients, you
will see some implausible jumps from one to another. If we wish to smooth them out, we could
impose constraints like the following:
     con   100   0   =   a3 - 2a4 + a5
     con   100   0   =   a4 - 2a5 + a6
     con   100   0   =   a5 - 2a6 + a7
     con   100   0   =   a6 - 2a7 + a8
     con   100   0   =   a7 - 2a8 + a9
     con   100   0   =   a8 - 2a9 + a10
     con   100   0   =   a9 - 2a10 + a11
     con   100   0   =   a10 - 2a11 + a12
     con   100   0   =   a11 - 2a12 + a13
     con   100   0   =   a12 - 2a13 + a14
     con   100   0   =   a13 – 2a14

where it is assumed that a15 is zero.



                                                                                                       95
Now it is a bit tiresome to type all of these lines, so G7 provides a shorthand, the “sma” command:
     sma 100 a3 a14      1


This one command is the exact equivalent of the eleven “con” commands. The 100 serves the same
purpose as the 100, the “count” parameter, in the “con” commands. The a3 and a14 are the first and
last coefficients in the sequence of “con” commands. The “con” commands are imposing the
constraint that the coefficients lie on a straight line, which is a polynomial of degree 1; the one 1 at
the end of the sma command indicates that it also should impose softly a straight line. If the 1 were
replaced by a 2, 3, or 4, etc., the “sma” would impose softly a polynomial of degree 2, 3, or 4,
respectively.

The “sma” command as given above includes the presumption that a15 is zero, so that a13, a14, and
a15 = 0 should lie on a straight line; or, as in the last of the “con” commands, 0 = a13 - 2a14, or
a14 = .5a13. If this constraint is not desired, then the command should end with an “f” , thus
     sma 100 a3 a14 1 f


The “f” is for “free” or “floating”, not tied down to zero at the end.

Why the name “sma”? The “sma” is a “soft” version (suggested by Robert Shiller) of the lag
estimation system introduced by Shirley M. Almon, my late wife, and generally known as the Almon
lag. By picking sufficiently large values of the “count” parameter, one can use the “sma” command
to produce the original Almon lag, though I think it is seldom desirable to do so. Several regression
packages have a function called “Almon” which implements this lag. Since I wished to be clear that
this was Shirley’s work, not mine, I have used her initials as the name of the command.

Applied to the present equation for inflation, the sma command equivalent to the three con
commands given above is
     sma 20 a3    a6 1


The numerical results are quite satisfactory:
     :                                    Inflation 4
         SEE    =      0.66 RSQ   = 0.9024 RHO =     0.87 Obser = 123 from 1977.100
         SEE+1 =       0.33 RBSQ = 0.8965 DW =       0.26 DoFree = 115 to    2007.300
         MAPE =       16.97
           Variable name           Reg-Coef Mexval Elas       NorRes    Mean    Beta
         0 infl                  - - - - - - - - - - - - - - - - -        3.53 - - -
         1 intercept                1.88959    16.8     0.54   46.15      1.00
         2 inflex                   0.92056   544.3     1.00    2.49      3.84 0.866
         3 u                       -0.12294    10.0 -0.21       2.45      6.13 -0.081
         4 u[1]                    -0.09679    16.1 -0.17       2.24      6.15 -0.064
         5 u[2]                    -0.06841    10.9 -0.12       2.04      6.18 -0.045
         6 u[3]                    -0.03604      4.9 -0.06      1.88      6.20 -0.024
         7 inflimp[2]               0.05142      1.1    0.01    1.15      0.53 0.086
         8 inflimp[3]               0.13428      7.2    0.02    1.00      0.53 0.225

The coefficients on u are now quite sensible and the cost in terms of a worse fit is quite small; SEE

96
rose from .96 to 1.00. The graph of the fit is virtually indistinguishable from that of the Inflation III
equation. The graph on the right below shows the simulation.

                            Inflation 4                                                                                    infl - Inflation 4
                            Regression Fit                                                                                  Historical Simulation
   9.77                                                                                      9.77




   5.39                                                                                      5.40




   1.02                                                                                      1.04

          1980       1985   1990         1995                2000            2005          1980             1985          1990          1995        2000   2005
     Predicted   Actual                                                                           b.infl       c.infl



                                                                                    GDP Deflator
                                                                                    Historical simulation
                                               1.44




                                               0.98




                                               0.52

                                             1980              1985             1990               1995            2000          2005
                                                    b.gdpD          c.gdpD




The model needs at various points the price level, gdpD, as well as the inflation rate. We work back
from the inflation rate to the price level by the equations
      f lgdpD = lgdpD[4] + infl
      f gdpD = @exp(.01*lgdpD)


The historical simulation for gdpD is shown in the third graph of the group just above. In the present
case, this simulation is acceptable only until 1987. It should be obvious that the procedure we have
used thus far can produce results for the historically simulated gdpD which deviate steadily more
and more from the historical value.

This deflator is as important to us as is the rate of inflation, indeed, perhaps more so. For it is what
we shall use to convert gdpR to gdp. And it is the ratio of gdp (in current prices) to some measures
of the money supply which will indicate credit conditions. Thus, a systematic error in gdpD is a
cause of considerable concern.



                                                                                                                                                                  97
     8. Inflation V: Working in Levels instead of Rates of Change

How can we get the equation to work well for gdpD? Well, the obvious answer is to estimate the
equation for gdpD (or lgdpD — its logarithm) instead of for infl. How can we do that? First just
note that inflation by the logarithmic definition is the first difference of lgdpD, so lgdpD is just the
cumulate of inflation. That being the case, we can convert our equation to work in levels by just
cumulating with 0 spill rate all the variables which appear on the right side. Because these inflation
rates are expressed as annual rates while the cumulation is done quarterly, the annual rates have been
multiplied by 0.25 before cumulation. The G7 commands to do so are just
     f cinflex = @cum(cinflex,0.25*inflex,0.)
     f cu = @cum(cu, 0.25*u, 0)
     f cinflimp = @cum(cinflimp,0.25*inflimp,0)


and the cumulate of the constant term is just a time trend. Thus, the regression command in G7 is
     r   lgdpD = cinflex, cu,cu[1],cu[2],cu[3],cu[4], cinflimp[2],
          cinflimp[3], time

and the commands to compute from lgdpD the deflator and inflation (which should immediately
follow the below the r command) are
     f gdpD   =   @exp(.01*lgdpD)
     f infl   =   lgdpD - lgdpD[4]

The constraint on the coefficient of expected inflation became unnecessary and was removed. The
others were left unchanged. The numerical results are


     :                                   Inflation 5
       SEE    =       0.72 RSQ   = 0.9995 RHO =     0.83 Obser = 131          from 1975.100
       SEE+1 =        0.41 RBSQ = 0.9994 DW =       0.34 DoFree = 121         to   2007.300
       MAPE =        10.39
         Variable name            Reg-Coef Mexval Elas       NorRes            Mean   Beta
       0 lgdpD                  - - - - - - - - - - - - - - - - -             -26.07 - - -
       1 intercept              -147.80219   773.3     5.67 2075.11             1.00
       2 cinflex                   1.11371   221.7 -4.12 135.86                96.46 1.301
       3 cu                        0.79961 1022.9 -17.13 130.12               558.40 5.996
       4 cu[1]                    -2.39589    25.6 50.74       8.79           552.15-18.042
       5 cu[2]                     1.47981      1.6 -30.99     8.78           545.89 11.189
       6 cu[3]                     0.92744      0.7 -19.20     8.73           539.63 7.041
       7 cu[4]                    -0.99310      5.4 20.32      8.73           533.35 -7.568
       8 cinflimp[2]               0.43525    12.1 -0.56       3.37            33.50 0.070
       9 cinflimp[3]              -0.11595      0.9    0.15    3.37            33.33 -0.020
      10 time                      3.55673    83.6 -3.89       1.00            28.50 1.065




98
                                             Inflation 5                                                                                        GDP Deflator
                                             Regression Fit                                                                                     Historical simulation
       78526712                                                                                             1.31




       39263356

                                                                                                            0.92




          0                                                                                                 0.52

   1975               1980            1985     1990               1995          2000      2005           1980               1985            1990            1995        2000   2005
          Predicted          Actual                                                                             b.gdpD          c.gdpD




                                                                                        infl - Inflation 5
                                                                                         Historical Simulation
                                                        12.1




                                                        6.5




                                                        0.8

                                                      1980               1985          1990          1995                2000            2005
                                                             b.infl         c.infl




The historical simulation of the deflator, gdpD still rises too slowly. However, this forecast is clearly
superior to the previous ones. This last lesson is an important one. It matters whether we work in
levels or differences. In general, my experience is that it is best to work at the level of the most
“integrated” concept that we want to use, in our case, the levels of prices, not the rate of inflation.
The trouble with working at the “differenced” level, inflation in our case, is that if we miss on, say,
the low side a few times, then the cumulated function gets on the low side and many periods may go
by before it catches up.

                                                                                                 Exercise

5.1.            Include in your current version of the AMI model an equation for the price level. First test
               the equation by itself by defining unemployment with a “fex” command. Then replace the
               “fex” with just “f” so that unemployment becomes endogenous. Then rerun your
               experiments with increasing gR and watch what happens to inflation.




                                                                                                                                                                                      99
      9. Ways to Improve AMI

Addition of the inflation equation to AMI began a process of making the model more comprehensive.
You can now carry that process further in many directions. Here are some possibilities.

Improve the Investment Equations.

The graphs of the historical simulations of AMI showed large errors in the simulation of total fixed
investment. The effects of these errors on the rest of the model can be seen by running a historical
simulation with the statement “skip vfR” in the .fix file. This statement will cause the model to use
the actual value of vfR rather than the equation’s value. The improvement in the model’s
performance is striking. The first step in improving the fixed investment equation is certainly to
make use of the distinction which the national accounts offer among three different types of fixed
investment: (1) Residential investment (2) Nonresidential structures and (3) Producers’ durable
equipment. Residential investment should depend heavily on personal disposable income, interest
rates, and the stock of housing created by a bucket. Do not be surprised if this stock of housing has a
negative sign. The negative coefficient would simply mean that if we have housing, we don’t need
to build it. Equipment investment can follow very much the pattern of the equation we used in AMI
for all investment, but with the stock formed from only equipment investment and with a faster spill
rate than was used for total investment.. Nonresidential structures are apt to be quite a problem,
probably because of their great diversity, from electric power plants and oil pipelines to churches and
swimming pools.

Improve the Savings Function.

AMI uses exogenous behavioral ratios for Personal savings, Interest paid by persons to business, and
Personal transfers to foreigners. That approach implies that these ratios are unaffected by, say, the
interest rate, a most unlikely result. Now is a good time to replace the exogenous ratios by
economically meaningful relations.

The identity relating income, consumption, interest payments, and savings points out that an increase
in interest payments relative to income can come out of either consumption or savings. Thus, the
best place to begin work may be on these interest payments. Interest rates, automobile and other
durable purchases would be good candidates for explanatory variables, but clearly you do not expect
an additive relationship nor one involving just current values of interest rates and durable purchases.
With interest payments explained, the savings ratio would be the next in line. Here also interest
rates might be expected to play a role, along with the ratio of piipcb to pidis. It has also been
suggested that the ratio of the contributions to social insurance (Social Security) to disposable
income might play a role. You may also find that automobile purchases in the current quarter help
explain savings, for the accounts consider that you totally consume the car the moment you buy it.
Thus, a quarter of unusually high automobile expenditures might well prove to be one of low
savings. But if you use automobile purchases (cdmvR), you will need to add an equation to explain
it.

The remaining variable in the income-consumption-savings identity, personal transfers to foreigners

100
(pipttf), is small and could be left as an exogenous ratio.

Expand the financial sector.

So far, money supply has played no role in the model and interest rates, if used, have remained
exogenous. The monetary policy variable, however, is not interest rates — which are strongly
influenced by inflation — but some measure of the money supply. The Quip data bank has several
measures of money supply including M1 and M2. They are natural exogenous variables for the
model, though exact modeling of them from the variable the Federal Reserve’s open market
operations control directly, the unborrowed reserves, is itself a tricky matter. Begin your study here
by graphing the ratios of each of these measures of money supply to GDP. With the results of those
graphs clearly in mind, develop an equation for the rate on 90-day Treasury bills, rtb. Remember
that expected inflation plays a big role in determining interest rates.

Expand the Government Sector

The government sector in AMI is both highly aggregated and incomplete. Section 3 of the NIPA give
the accounts of Federal and of State and local governments separately. Include these accounts in the
model so that it can calculate the deficit of the Federal government and the surplus or deficit of the
State and local governments. You will probably want to leave many of the expenditures exogenous
in constant prices; interest payments will certainly depend upon accumulated deficits and interest
rates. Likewise, unemployment insurance costs depend on the level of economic activity. On the
revenue side, you may want to make tax rates exogenous while the tax bases are endogenous. AMI’s
one taxrate variable is an example, but you should have a number of rates for different taxes.

Improve the Foreign Sector

You can add an equation for exports as a function of foreign demand, fgndem in Quip. This variable
is constructed as a weighted average of total real imports of our trading partners. By looking at our
exports relative to it, we can judge whether we are gaining or losing shares in world markets. You
will find that the change has been considerable. Can you explain it by changes in relative prices?
You have in Quip exports in current and constant prices (x and xR) and the same for imports (m and
mR). You may wish to exclude petroleum from the price index for imports, and you have the
necessary material for doing so in Quip. You may also wish to revisit the import equation and put in
variables for relative prices. And, of course, if you find these relative price variables valuable, you
may want to develop equations for them also.




                                                                                                   101
102
                     CHAPTER 6. GOOD ADVICE AND BAD EXAMPLES

    1. Good Advice

How can you tell a good equation from a bad one? If there were any single rule, it would not be
necessary for G7 to print out its many statistics and graphs. It could just report the coefficients and
give some numerical grade to the equation. Students who themselves are going to receive a silly
numerical or alphabetical grade on their equation want to know what is expected. Unfortunately,
there are absolutely no absolute answers to that question. Judging an equation requires thought.
What G7 offers is food for thought but is no substitute for thought itself. In this chapter, we shall
discuss some ways of thinking about equations.

Since R2 is used to measure the closeness of fit, a beginner often assumes that R2 is also the measure
of his success in developing an equation. "How high should my R2 be?" is frequently asked, but
there can be no general answer. For some items, say inventory change, .50 may be quite
satisfactory; for others, say consumption of food, anything below .98 would be embarrassing. If R2
alone, then, is inadequate as a guide, what else should be considered in judging the quality of an
equation? Here are some things a good equation should do.

1. Account for important influences. If the equation is for food consumption, it should certainly
account for the influences of income and the price of food relative to other commodities. On the
other hand, if it fails to include the percentage of women in the labor force, or the stock of home
appliances, or the stock of housing, or the price of electricity and gas, it can hardly be seriously
faulted. In fact, if one of these other factors turned out to be crucial, one should be suspicious.

2. Economize on variables. In forecasting, every independent variable must either be produced in
the model or must be exogenously forecasted. Be sparing, therefore, in the use of special-purpose
variables. Especially avoid variables whose forecasting is tantamount to forecasting the dependent
variable. For example, the Federal Reserve discount rate would prove a very good variable for
explaining the Treasury bill rate in the QUEST model. Since the model does not, however, otherwise
explain the discount rate, an exogenous forecast of it would be required, and making an exogenous
forecast of the discount is just about the same thing as forecasting the T-bill rate. Do not clutter up
the equation with numerous variables of small marginal explanatory value (mexval), for each
unnecessary variable reduces the efficiency with which the coefficients on the others are estimated.
Remember Occam's razor, and prefer a simple equation to a complicated one. On the other hand, if
there are strong reasons for believing that a variable should enter the equation and if in fact it does so
and with a plausible coefficient, do not throw it out just because its mexval is low or its t-statistic is
less than 2.0 in absolute value.

3. Avoid false exogeneity. You can very easily eliminate the excessive cycles in AMI by making
investment depend on changes in real National income instead of changes in real Gross private
product. Why? Because real National income is not otherwise in the model, so it is technically an
exogenous variable in the model. Consequently, when the model is run in historical simulation it
will always have exactly the historical values of National income, and the predicted values for
investment will be those found when the equation was fit. But National income is closely related to

                                                                                                      103
GDP and cannot rightfully be considered exogenous. If you run an experiment by increasing, say,
exports, you will get no impact on investment because the exogenous National income is not
affected although GDP is affected. In the logic of the national accounts, National income should
also be affected, but because you have left it exogenous, it is not. When it comes to forecasting, you
will have to produce an exogenous forecast of National income. If this forecast turns out to be out of
line with the GDP forecast, you will have a problem. All these problems arise because of what I call
the false exogeneity of National income. Avoid all such false exogeneity. Limit the exogenous
variables to policy variables, such as tax rates or government expenditures, or variables truly
unaffected by short-run economic fluctuations, such as population.

4. Use appropriate dimensions. If the dependent variable is, say, food expenditure per capita, then
it would be inappropriate to use total disposable income to explain it. Total income might grow
because total population was growing but without any change in income per capita to increase food
purchases per capita. Similarly, one would not use the total population aged 16-25 in this equation,
though the fraction of the population in that age range might be quite appropriate. Or again, the
index of food prices would be inappropriate -- because food is not the only thing whose price has
gone up -- but the ratio of food prices to an overall price index could be used. Failure to get
appropriate dimensions is a common problem with student regressions.

5. Mix trend and stationary variables only with utmost caution. Roughly speaking, a trend
variable is one which, you know without plotting, would, if plotted, go from the lower left to the
upper right of the screen. GDP, population, the consumer price index, and the quantity of money are
familiar examples. A stationary series, again roughly speaking, is one where you cannot count on
such a trend. The growth rate of GDP or quantity of money, an interest rate, the price of imports
relative to domestic goods, the price of gasoline relative to the price of shoes, or rainfall or mean
temperature for the year are typical examples. If a stationary variable, say the interest rate, is used to
explain a trended variable, say the level of aggregate investment, then one has built into the very
form of the equation a declining relative importance for the interest rate as the economy -- and with
it, aggregate investment -- grows. Conversely, if one tries to explain a stationary variable, say an
interest rate, with a trended variable, say the quantity of money, one is clearly looking for a relation
which cannot exist. This simple fact is the valid point behind the recent vogue for studying
"cointegration".

6. Avoid "umbrella" variables. Once I was criticizing the equation of a student who had used
residential construction to explain interest rates. High levels of residential construction seemed to
cause interest rates to fall. When the student finally understood my objections, he exclaimed, "Oh, I
see. It is as if I had used the number of people carrying umbrellas to explain rainfall." Exactly. The
independent variables should "cause" the dependent variable, not vice-versa. While the question of
what "cause" means in economics is not always clear, a little common sense applied to eliminating
such "umbrella" variables will pay good dividends.

7. Allow for lags as necessary. Any relevant variable may act with a lag or a distributed lag. Those
lags should be looked for, and if found, the lag distribution should be encouraged, via buckets or soft
constraints, to take on a sensible shape. On the other hand, lagged values of the dependent variable
should not be used to express lags, since their use will generally distort the parameters on other

104
variables and lead to excessively long lags.

8. Have plausible parameter values. Try your best to interpret every parameter in intuitive terms.
Of every regression coefficient (except perhaps the intercept) one should ask "Does that value for
this coefficient make sense?" If the answer is "no," there may be a case for using soft constraints.
For some equations, it is the elasticities which should make sense. To continue the food example,
one would expect the income elasticity to be positive but definitely less than 1.0, while the price
elasticity should be negative and probably less than 1.0 in absolute value. (In other equations -- and
especially for variables with means close to zero -- the elasticities have no intuitive value.)

9. Fit adequately. Look at the plot of the regression. Has the equation explained the trend and the
major variations from the trend? An equation that misses by a wide margin on the same side for
years at a time is missing something important. Try to think what might be the cause and to find a
variable to express that cause. Note that this point comes near the end of the list, not at the top.

10. Show stability of the coefficients when the sample period is changed. One way to check for
this stability is to try the equation with a test period of two or three years at the end of the fit period.
Another is through the use of the "recur" command in G7. If the coefficients change significantly
when the period of fit is changed, then they probably do not reflect a true structure but just an
accident of the period you are looking at. Both of these techniques are illustrated in the next section.
The lever variable should also be checked mainly to spot spurious observations.

Note that the first six points can be and should be taken care of before you ever try to estimate the
equation. They are the most important for the functioning of the model.

Until these criteria become second nature, you should use them as a checklist for each equation in
your model.


    2. Bad Examples

Since the QUEST model contains numerous examples of more or less good equations, we will take
the opposite approach here and show some examples of equations that are definitely bad. Except for
the last, we should not immortalize these bad examples with beautiful graphs, so I will just show the
setup of the equations and the numerical results.

Consumption depends on population, so we could do the following:
     title Consumption and Population
     r c = pop


with these results:




                                                                                                        105
      :                            Consumption and Population
          SEE   =     285.38 RSQ   = 0.9843 RHO =   0.99 Obser = 111 from 1980.100
          SEE+1 =      44.01 RBSQ = 0.9842 DW =     0.02 DoFree = 109 to    2007.300
          MAPE =        4.92
            Variable name           Reg-Coef Mexval Elas      NorRes   Mean    Beta
          0 c                     - - - - - - - - - - - - - - - - -   4961.84 - - -
          1 intercept            -20811.01942   545.6 -4.19     63.87    1.00
          2 pop                   98111.91900   699.2   5.19     1.00    0.26 0.992


The RSQ is terrific, although the MAPE leaves room for improvement. But look at the elasticity of
population. A one percent increase in population seems to cause a five percent increase in
consumption. How can that be? One problem is certainly that we have used current dollar
consumption so that we are, in effect, using population to explain inflation. We have, in my terms,
inappropriate dimensions. The next equation reduces that problem by using consumption in constant
prices, like this:
      title Real Consumption and Population
      r c$ = pop

with these results:
      :                          Real Consumption and Population
          SEE   =     165.50 RSQ   = 0.9872 RHO =   0.98 Obser = 111 from 1980.100
          SEE+1 =      35.06 RBSQ = 0.9871 DW =     0.03 DoFree = 109 to    2007.300
          MAPE =        2.65
            Variable name           Reg-Coef Mexval Elas     NorRes    Mean    Beta
          0 c$                    - - - - - - - - - - - - - - - - -   5464.50 - - -
          1 intercept            -11118.45638   496.0 -2.03    78.39     1.00
          2 pop                   63127.84857   785.4   3.03     1.00    0.26 0.994

The MAPE is better and the SEE is lower, and the RSQ is slightly better. The elasticity of
consumption with respect to population is lower but still unreasonably high. The problem is that we
have not accounted for some important influences, notably increasing income. So we could throw in
real disposable income like this.
      title Real Consumption, Population and Income
      r c$ = pop, pidis$

      :                      Real Consumption, Population and Income
          SEE   =      53.17 RSQ   = 0.9987 RHO =    0.55 Obser = 111     from 1980.100
          SEE+1 =      45.31 RBSQ = 0.9987 DW =      0.91 DoFree = 108    to   2007.300
          MAPE =        0.83
            Variable name           Reg-Coef Mexval Elas      NorRes       Mean   Beta
          0 c$                    - - - - - - - - - - - - - - - - -       5464.50 - - -
          1 intercept             -1600.76056     11.2 -0.29 759.36          1.00
          2 pop                    5265.53987      3.5   0.25    9.69        0.26 0.083
          3 pidis$                    0.95355   211.2    1.04    1.00     5958.87 0.917


Now the elasticity on population is deceptive. It seems to say that if population increases by one
percent, consumption will increase by .25 percent. But that calculation assumes that income remains
constant. If income also goes up by one percent, so that per capita income stays constant, then
consumption goes up by .25 + 1.04 = 1.29 percent. That is certainly better than the previous


106
equations. But what we would probably really like to see is an equation which makes consumption
go up exactly one percent when population goes up one percent and per capita income is unchanged.
We can impose that requirement by working with per capita income. Does this equation do the
trick?
     title Consumption and Income per Capita
     f ypc$ = pidis$/pop
     r c$ = ypc$

     :                          Consumption and Income per Capita
         SEE    =    146.24 RSQ    = 0.9900 RHO =    0.84 Obser = 111 from 1980.100
         SEE+1 =      81.61 RBSQ = 0.9899 DW =       0.31 DoFree = 109 to    2007.300
         MAPE =        2.68
           Variable name            Reg-Coef Mexval Elas      NorRes    Mean    Beta
         0 c$                     - - - - - - - - - - - - - - - - -   5464.50 - - -
         1 intercept             -4255.13721   343.1 -0.78 100.40         1.00
         2 ypc$                      0.43405   902.0    1.78    1.00 22393.13 0.995

No, it does not. It has violated the "appropriate dimensions" maxim again because it is relating total
consumption to per capita income. Here is the equation we should have estimated.
     title Consumption per Capita and Income per Capita
     f cpc$ = c$/pop
     r cpc$ = ypc$

     :                   Consumption per Capita and Income per Capita
         SEE    =    230.10 RSQ   = 0.9962 RHO =   0.57 Obser = 111 from 1980.100
         SEE+1 =     191.42 RBSQ = 0.9961 DW =     0.86 DoFree = 109 to    2007.300
         MAPE =        1.00
           Variable name           Reg-Coef Mexval Elas     NorRes    Mean    Beta
         0 cpc$                  - - - - - - - - - - - - - - - - - 20478.20 - - -
         1 intercept             -4301.16239   194.8 -0.21 261.95       1.00
         2 ypc$                      1.10656 1518.5    1.21    1.00 22393.13 0.998

The elasticity of consumption with respect to population -- with per capita income constant -- is 1.0
by definition with this equation. The elasticity of consumption per capita with respect to income per
capita turns out to be somewhat greater than 1.0. That is because the equation mistakenly attributes
the decline in the savings rate in the United States to increasing income. The decline had other
causes, which we will discuss in connection with the QUEST model in Part 2.

Although this last equation passes the maxims of this chapter fairly well, it probably suffers from
more subtle problems. If, for some reason, consumption is higher in a particular quarter than it
"should" have been according to the equation, then -- because consumption influences income --
income is probably unusually high in that quarter. Thus, the error term in this equation is probably
correlated with the independent variable. This problem is referred to as "simultaneous equation
bias" or sometimes (less accurately) as "least squares bias." What to do about it is dealt with in Part
2.

Here is an example adapted only slightly from a recent major work of a major economist. It begins
from the idea that consumption expenditures per capita on automobiles depends on income per
capita and the price of automobiles. The price of automobiles is expressed by taking the ratio of
automobile purchases in current prices to the same thing in constant prices. The example goes as


                                                                                                         107
follows:
      f   pauto = cdmv/cdmv$ # price of automobiles
      f   cdmvpc$ = cdmv$/pop
      f   ypc$ = pidis$/pop
      r   cdmvpc$ = ypc$, pauto

Both variables turn out to be highly significant and the elasticity on pauto comes out to be about -.2.
The elasticity on income, however, is over 1.5. How can that be? Do you see what the problem is
without reading further?

The problem lies, of course, with the pauto variable. As calculated, it is an index of automobile
prices, not the price of automobiles relative to other competing goods. The variable which should
have been used was
      f pauto = (cdmv/cdmv$)/(c/c$)

Try the regression with this variable. You should be quite unhappy with the results, for they show a
positive price elasticity. I'll not pursue this equation further here since a full treatment is given later
with the QUEST model in Part 2. Suffice it here to emphasize the importance of thinking about the
whole structure of the equation and considering the plausibility of the magnitudes of each
coefficient, rather than just looking at the correctness of the signs on the coefficients.

There are a number of ways of fooling yourself into thinking that you have a good equation when, in
fact, what you have is trash. Here is one way. We need an equation for construction of non-
residential structures, vfnrsR, and we suppose that it might be proportional to the difference between
need for such structures, which we take to be proportional to real GDP, gdpR, and the stock of such
structures. So we make up an equation like this:
      ti vfnrsR - Non-residential Structures
      lim 1980.1 2007.3
      f ub01 = @cum(ub01,1.,.01)
      # Make up stock of structures
      f StockSt = @cum(cumSt,vfnrsR,.01)/ub01
      r vfnrsR = gdpR,StockSt

It doesn’t work very well, so let’s try lagging the stock of structures one quarter with this regression
command:
      r vfnrsR = gdpR,StockSt,StockSt[1]

Now we get the following wonderful results:
      :                        vfnrsR - Non-residential Structures
          SEE    =      6.82 RSQ   = 0.9726 RHO =     0.94 Obser = 111 from 1980.100
          SEE+1 =       2.40 RBSQ = 0.9718 DW =       0.12 DoFree = 107 to    2007.300
          MAPE =        1.93
            Variable name           Reg-Coef Mexval Elas       NorRes    Mean    Beta
          0 vfnrsR                - - - - - - - - - - - - - - - - -     263.25 - - -
          1 intercept             -207.54437    42.4 -0.79      36.46      1.00
          2 gdpR                     0.00053      0.2    0.02   26.15  7985.15 0.025
          3 StockSt                 54.55124   390.6 46.45      25.16   224.16 21.725



108
       4 StockSt[1]                          -52.64073         401.6 -44.68          1.00      223.44-21.345



                                               vfnrsR - Non-residential Structures
                           401




                           299




                           197

                         1980               1985        1990      1995       2000       2005
                                Predicted      Actual




The fit is fantastic; the mexval’s are enormous; what could be wrong? First of all, we used the
current value of the dependent variable in making up the StockSt variable; that will cause problems
in forecasting. Now, without the unit bucket adjustment, the current value of the stock is just the
dependent variable plus 99 percent of the lagged value of the stock. So the regression can just turn
that identity around to compute the dependent variable from the current and lagged values of the
stock. The use of the unit bucket adjustment blurs the identity just enough that we may not
recognize immediately what a silly thing we have done. Certainly, the equation is utterly worthless
for forecasting or counter-historical simulation.

If we define the stock using the lagged value of the dependent variable, as with the command

     f StockSt = @cum(cumSt,vfnrsR[1],.01)


then the regression can reconstruct this lagged value from StockSt and StockSt[1]. That lagged value
of the dependent variable, as we will see in the next section, is one of the most deceptive,
destructive, and dangerous variables you can put into an equation with which you hope to capture
structure.

In teaching the uses of regression, the first problem is to get students to think of possible variables
that could be used to explain a given variable. Then, once they catch on to that process, the problem
is to bring them under control. It is not unusual to encounter regressions like this one:
     save vfnrsR.sav
     ti vfnrsR - Non-residential Structures
     lim 1975.1 2001.4
     f d = gdpR - gdpR[1]
     f ub01 = @cum(ub01,1.,.01)
     f StockSt = 100.*@cum(cumSt,0.25*vfnrsR[1],.01)/ub01


                                                                                                               109
      f m1R = m1/gdpD
      f niprfR = niprf/gdpD
      r vfnrsR = gdpR[1],d[1],d[2],d[3],d[4],d[5],d[6],d[7],
      d[8],d[9],d[10],
      StockSt[1],m1R, m1R[1],rtb,rtb[1], niprfR, niprfR[1]

      :                      vfnrsR - Non-residential Structures
        SEE    =     11.80 RSQ   = 0.9181 RHO =     0.79 Obser = 111 from 1980.100
        SEE+1 =       7.53 RBSQ = 0.9020 DW =       0.43 DoFree =   92 to    2007.300
        MAPE =        3.51
          Variable name           Reg-Coef Mexval Elas       NorRes     Mean    Beta
        0 vfnrsR                - - - - - - - - - - - - - - - - -      263.25 - - -
        1 intercept               77.36028      1.3    0.29   12.21       1.00
        2 gdpR[1]                  0.02378    29.4     0.72    8.71   7927.03 1.118
        3 d[1]                    -0.01244      0.1 -0.00      8.54     57.01 -0.015
        4 d[2]                    -0.01753      0.2 -0.00      8.52     56.43 -0.021
        5 d[3]                     0.02152      0.3    0.00    8.43     56.34 0.026
        6 d[4]                     0.05813      2.0    0.01    8.30     55.87 0.069
        7 d[5]                     0.09081      4.1    0.02    8.02     56.24 0.108
        8 d[6]                     0.03713      0.8    0.01    7.88     56.04 0.044
        9 d[7]                     0.01994      0.3    0.00    7.81     56.52 0.024
       10 d[8]                     0.06798      2.7    0.01    7.70     56.28 0.083
       11 d[9]                     0.03877      1.0    0.01    7.68     55.41 0.047
       12 d[10]                    0.01649      0.2    0.00    7.64     55.42 0.020
       13 StockSt[1]               0.01650      2.0    0.35    6.59   5554.78 0.172
       14 m1R                      0.22629      1.3    0.90    1.61   1045.07 0.934
       15 m1R[1]                  -0.41130      4.9 -1.63      1.58   1041.49 -1.719
       16 rtb                      1.87863      0.6    0.04    1.19       5.81 0.141
       17 rtb[1]                   6.59545      6.8    0.15    1.09       5.88 0.503
       18 niprfR                   0.06644      1.5    0.18    1.00    707.89 0.448
       19 niprfR[1]               -0.02440      0.2 -0.06      1.00    699.58 -0.161

                                               vfnrsR - Non-residential Structures
                           401




                           293




                           186

                         1980               1985        1990      1995       2000    2005
                                Predicted      Actual




The fit is undeniably excellent. The negative sign on M1 lagged and the positive signs on the
interest rate should give one pause. But I have often been more or less told by the ingenious and
proud creators of such equations, “How do you know what those signs ought to be? Look at this
fit!”



110
Well, one way that often shows that warnings of theoretically wrong signs should have been heeded
is to fit the equation over only part of the period and to use the end of the data to test how the
equation performs outside the period over which it was fit. Let’s try this with the above equation.
We change only the “limits” line to

         lim 1980.1 1995.4 2007.3


The fit over the shorter period is only slightly worse:
     :                      vfnrsR - Non-residential Structures
       SEE    =      8.91 RSQ   = 0.8961 RHO =     0.59 Obser =     64 from 1980.100
       SEE+1 =       7.34 RBSQ = 0.8545 DW =       0.83 DoFree =    45 to    1995.400
       MAPE =        3.12 Test period:    SEE   118.04 MAPE      36.98 end 2007.300
         Variable name           Reg-Coef Mexval Elas       NorRes      Mean    Beta
       0 vfnrsR                - - - - - - - - - - - - - - - - -       241.71 - - -
       1 intercept               45.08512      0.6    0.19    9.62        1.00
       2 gdpR[1]                 -0.02315      4.6 -0.62      4.60    6474.78 -0.772
       3 d[1]                    -0.03337      0.9 -0.01      4.55      44.73 -0.058
       4 d[2]                    -0.00790      0.1 -0.00      4.47      44.32 -0.014
       5 d[3]                     0.03836      1.4    0.01    4.07      44.22 0.067
       6 d[4]                     0.09109      7.2    0.02    3.54      44.01 0.159
       7 d[5]                     0.11396      8.5    0.02    3.12      43.68 0.197
       8 d[6]                     0.00980      0.1    0.00    2.99      43.68 0.017
       9 d[7]                     0.01982      0.4    0.00    2.93      45.02 0.036
      10 d[8]                     0.06395      4.3    0.01    2.89      43.95 0.117
      11 d[9]                     0.04143      1.9    0.01    2.88      42.67 0.075
      12 d[10]                    0.00137      0.0    0.00    2.87      43.30 0.002
      13 StockSt[1]               0.03920     11.6    0.86    2.87    5305.29 0.511
      14 m1R                      0.21987      1.5    0.88    1.89     963.03 1.445
      15 m1R[1]                  -0.18784      1.3 -0.74      1.74     955.60 -1.228
      16 rtb                      1.97907      1.3    0.06    1.37        7.33 0.218
      17 rtb[1]                   7.75587     16.8    0.24    1.03        7.43 0.865
      18 niprfR                   0.05863      1.1    0.12    1.00     510.27 0.235
      19 niprfR[1]               -0.02100      0.1 -0.04      1.00     504.81 -0.080


but the forecast into the test period is pathetic, as you can see from the SEE and MAPE in the test
period in the table above or even more dramatically in the graph below.




                                                                                                      111
                                               vfnrsR - Non-residential Structures
                           401




                           267




                           133

                         1980               1985        1990      1995       2000    2005
                                Predicted      Actual




Another way to check the reliability of a regression is through the use of recursive regression by
G7’s “recur” command. This amounts to running the same regression a number of times with
progressively longer periods of fit and observing what happens to the values of the estimated
regression coefficients. In G7, we need make only two minor changes is the commands to do
recursive regression. Just change
      lim 1980.1 2007.3
      ...
      r vfnrsR = gdpR[1],d[1],d[2],d[3],d[4],d[5],d[6],d[7],
to
      lim 1980.1 1995.1 2007.3
      ...
      recur vfnrsR = gdpR[1],d[1],d[2],d[3],d[4],d[5],d[6],d[7],


The first regression will be over the period 1995.1 to 2007.3, the next over 1994.4 to 2007.3, and so
on until the last is over 1980.1 2007.3. The results will pour out on the blue screen, but to make it
easier to see what is happening, G7 enters the values of each coefficient as time series in its
workspace bank. These series are named b1, b2, b3, etc., up to the number of variables in the
regression. The value of a particular b in a particular period is the value of that regression
coefficient in the regression beginning in that period. Thus, b1 for 1995.1 is the value of the
intercept in the regression from 1995.1 to 2007.3. Also entered into the workspace are the standard
deviations of the coefficients, s1, s2, s3, etc., as computed by the standard formulas.

The results are best viewed by graphing each coefficient and two standard deviations on either side.
Here is a G7 command file which will do the job for the present example.
      do {f lb = b%1 -2*s%1;
         f ub = b%1 +2*s%1;
         ti Regression Coefficient %1;
         gname coef%1;
         gr lb b%1 ub} (1-19)




112
This command file uses the do command of G7 to repeat the same commands with substitution of
text; it is fully described in the G7 help files and need not detain us here.

What one would hope to find in these graphs is something not worse than the graph on the left below
for coefficient 7. Nearly all of the estimated values, the middle line, are within two standard
deviations of any other value on that line, by standard deviations calculated for any period. If all of
the coefficients had graphs like that of number 7 (the coefficient for d[5]), we could say that the
recursive regression examination had been passed satisfactorily.

                   Regression Coefficient 7;                         Regression Coefficient 13;
      0.19                                              0.50




      0.05                                              0.21




      -0.08                                            -0.08

     1980            1985                1990   1995   1980             1985               1990   1995
        lb    b7            ub                            lb   b13             ub




Unfortunately, that is by no means the case. A much more typical result is that for Coefficient 13 for
the stock of structures. Estimation beginning in 1981.1 gives a “significant” negative coefficient;
estimation beginning a few quarters sooner or later gives a positive coefficient. “How can that be?”
you may well ask. The answer can only be that there was no stable structure of the kind posited by
this regression. Thus, this equation fails examination by recursive regression and thereby illustrates
how the method can perhaps save us from using equations which would make embarrassing
forecasts.

Next, let us look at a problem of mixing trended and stationary variables. We can see it in the
problem of relating imports to domestic demand and exchange rates. Here is one approach using a
"terms of trade" (tot) variable defined as the ratio of the prices of imported goods relative to the
prices of domestically produced goods.
     ti m$ -- Imports
     " Define Domestic Demand
     f dd$ = c$ + v$ + g$ + x$
     " define terms of trade as ratio of foreign to domestic prices.
     f tot = (m/m$)/(gnp/gnp$)
     r m$ = dd$, tot


Over a long period, Domestic Demand (dd$) and Imports (m$) both grow substantially. The terms
of trade variable (tot), however, is essentially a relative price. It is unlikely to have a long-term,
secular trend. Consider a future year when dd$ will have doubled, and, let us say, m$ has tripled, but


                                                                                                         113
tot is about the same then as now. Will a change of, say .1, change m$ by the same dollar amount
then as now? The form of the equation requires an answer of "yes", but I think you will agree that
the correct answer should be "No, the effect will be larger." The problem is that we have mixed
stationary and trend variables.

One approach to the problem just posed is to regress the logarithm of m$ on those of dd$ and tot.
Then the elasticity of imports with respect to terms of trade will be constant. But we will have only
shifted the problem. If the elasticity of imports with respect to domestic demand is greater than 1.0
-- which is likely -- then with sufficient growth in domestic demand, imports will exceed domestic
demand and domestic production will have become negative. Here we have a case of what I call
asymptotic idiocy. One detects it by asking what happens to the equation as the variables move far
along the trends they are likely to follow. Actually, trend-stationary problems are just a special case
of it.

My long involvement in long-term forecasting has perhaps made me overly sensitive to the
asymptotic idiocy problem. It is certainly defensible to use for a short forecast an equation which is
asymptotically idiotic, but it does seem to me that one should be aware of the problem.
Unfortunately, avoiding asymptotically idiotic equations often requires non-linear equations with
coefficients which are laborious to estimate. We will see a method for doing so in Part 2.

If you are working on the improvement of AMI along the lines suggested at then end of Chapter 4,
you already have a number of equations to which to apply the ten-point check list of this chapter.


      3. The Worst Example

A lagged value of the dependent variable is frequently used as an independent variable not just by
greenhorn students of modeling but also by serious professionals who should know better. Such
equations are, for reasons I will explain, exercises in self deception. They are, however, frequently
excused with a comment to the effect that they allow for a distributed lag. Now allowing for a
distributed lag is, as we have seen, frequently a good thing to do. We therefore need to see first why
the lagged value of the dependent variable might possibly be a way to include a distributed lag and
then why it usually does something quite different.

Let us suppose, then, that y depends upon infinitely many prior values of x with exponentially
decreasing weights. The true equation relating y and x is then supposed to be
                    ∞
(1)       y t=ab ∑ i x t −i t
                   i=0


where 0 < λ < 1. At first glance, this equation seems to call for a regression of y on an infinity of
prior values of x. But the assumption of exponentially declining weights allows us to perform a trick
with the equation. First we lag the equation once and then multiply both sides by λ to get



114
                           ∞
(2)       y t−1= ab ∑ i x t−i t
                           i=1


Notice that the infinite summation on the right side of the two equations is the same except that the
summation in equation (2) lacks the first term of the summation in equation (1). Subtracting (2)
from (1) therefore gives

(3)          y t− y t −1 =a 1−bx tt − t −1

which we rewrite in a form to be estimated as

(4)          y t=b1b 2 x tb 3 y t −1 t − t −1

We can now estimate equation (4) without the slightest concern about an infinite number of
independent variables. The estimate of b3 is the estimate of λ. More flexible forms of lags can be
obtained by including more than one lagged value of the dependent variable. One should note, of
course, that if the error terms in equation (1) are independent, then the ones in equation (4) are
autocorrelated and correlated with one of the independent variables, namely yt-1 .

Now this transformation is indeed a clever trick and has mesmerized many model builders into a
rather indiscriminate use of lagged values of the dependent variable to account for lags. The
problem is that the equation with the lagged dependent variable may fit very well for reasons totally
unrelated to the supposed distributed lag on one particular variable. Namely, the dependent variable
may well depend upon many factors which change slowly relative to the period of observation. The
lagged value of the dependent variable sums up perfectly the effects of all these factors for one
period earlier. But, since the factors change slowly, that summary is still an excellent representation
of their values in the present period. Thus, the lagged value of the dependent variable will explain
most of the variation in the dependent variable and leave only a little noise to identify the
coefficients of the other variables in the regression.

Now equations with lagged values of the dependent variable offer an alternative way of calculating
the predicted value. Namely, instead of using the actual lagged value of the dependent variable in
computing the predicted value, we can use the value predicted for the first period to calculate the
prediction for the second period, and then use the predicted value for the second period to calculate
the prediction for the third, and so on. We will call the variable computed in this way BasePred,
because it uses the actual lagged value only in the base period. If G detects the lagged value of the
dependent variable, it automatically computes this BasePred and graphs it as a third line when the
“gr * command is given. If the above analysis is applicable to your equation, the fit of the ordinary
predicted value should be very close, while that of BasePred may be anywhere from so-so to
dreadful. Let us look at two examples.

In the first, we regress inflation on its own lagged value and the unemployment rate. The results are
shown below.2

2     If the lagged value of the dependent variable is included among the independent variables, the Durbin-Watson

                                                                                                                     115
      :                                      Inflation
          SEE    =      0.27 RSQ   = 0.9791 RHO =     0.43 Obser = 111 from 1980.100
          SEE+1 =       0.25 RBSQ = 0.9787 DurH = 4.62 DoFree = 108 to       2007.300
          MAPE =        6.87
            Variable name           Reg-Coef Mexval Elas       NorRes   Mean    Beta
          0 infl                  - - - - - - - - - - - - - - - - -      3.16 - - -
          1 intercept                0.49329      8.4    0.16   47.83    1.00
          2 u                       -0.08687      7.3 -0.17     35.67    6.11 -0.068
          3 infl[1]                  0.99517    497.2    1.01    1.00    3.21 1.026




                                                                      Inflation
                                                       Equation with Lagged Dependent Variable
                               9.77




                               4.97




                               0.17

                             1980               1985           1990          1995         2000   2005
                                    Predicted      Actual         BasePred




As you can see, the fit of the equation as normally calculated is fantastic; the fit of the BasePred line,
however, shows that the equation is worthless for forecasting beyond a few periods. It has failed to
identify the connection between unemployment and changes in inflation which our earlier equations
identified fairly well.

Another example is given by regressing Residential Construction on the real interest rate and two
lagged values of the dependent variable. The R2 is even better than before, and the performance on
the BasePred test is even more miserable. It doesn’t even do well right at the beginning.

      :                                           Residential Construction
          SEE   =          8.92 RSQ              = 0.9957 RHO = -0.01 Obser =                     111 from 1980.100
          SEE+1 =          8.91 RBSQ             = 0.9956 DurH = -0.15 DoFree =                   107 to   2007.300
          MAPE =           2.12

   statistic is invalid in the best of circumstances. Durbin devised another statistic for this situation known as the
   Durbin H. If the error terms are uncorrelated, the distribution of H approaches the normal with mean zero and unit
   standard deviation as the sample size grows. When G detects the presence of the lagged value of the dependent
   variable among the independent variables, it automatically replaces the Durbin Watson with the Durbin H.

116
           Variable name                        Reg-Coef Mexval Elas     NorRes                        Mean   Beta
       0   vfrR                               - - - - - - - - - - - - - - - - -                       368.61 - - -
       1   intercept                            15.12976     5.8   0.04 231.86                          1.00
       2   rtbR                                 -1.87157     4.8 -0.01 134.87                           2.66 -0.027
       3   vfrR[1]                               1.68168   160.2   1.67    2.04                       366.38 1.675
       4   vfrR[2]                              -0.70800    42.8 -0.70     1.00                       363.95 -0.700

                                                          Residential Construction
                                                     Equation with Lagged Dependent Variable
                             703




                             432




                             162

                           1980               1985           1990          1995         2000   2005
                                  Predicted      Actual         BasePred




Just because you get a close fit when using a lagged value of the dependent variable, do not suppose
that you have arrived at any kind of structural relation between the dependent and independent
variables. Possibly you have found such a relation, but much more probably you have simply found
that the determinants of the dependent variable change slowly from period to period. Those
determinants may or may not have anything to do with the independent variable you have specified.
If the dependent variable has a trend, BasePred will generally follow that trend, but that following is
no evidence of having found a structural relation.

When distributed lags are estimated by direct regression by putting a variable into a “bucket” (as we
did in creating inflex in the inflation examples of Chapter 5), and the independent variables fail to
explain the dependent, we immediately recognize that fact by poor fit. When the lagged value of the
dependent variable is slipped into the equation, we no longer have that warning. We can get
excellent fits without having found any causal relation between the independent and dependent
variables. I, therefore, find the lagged value of the dependent variable deceptive and avoid it. Self-
deception, however, is always popular, and the use of the lagged value of the dependent variable has
reached epidemic proportions. This use, I believe, is responsible for much poor forecasting and bad
policy analysis which has done nothing to engender respect for the economic forecasting profession
on the part of the general public.




                                                                                                                      117
118
                         CHAPTER 7. THE THEORY OF LINEAR SYSTEMS

      1. The general solution of a system of linear difference equations

In Chapter 1, we found that the AMI model had a pronounced tendency to cycle. Observers of the
economy have often thought they discerned cycles. Indeed, the term “business cycle” has become
part of the language. In modeling, our descriptions of the economy do not seem to have anything to
do with cycles. Where, then, do the cycles come from? In this short chapter, we shall try to answer
that question.

In this first section, we will seek the general solution of a system of difference equations such as can,
with some approximation, represent many of our models. It will be an abstract and analytical
section. In the following sections, that general solution will enable us see some interesting features
of models and, perhaps, of economies. I must emphasize, however, that this chapter is aimed at
understanding qualitatively the results of models, not at calculations you would ever be likely to
want to make. We will find, for example, the form of the general solution of a linear model. You
generally want, however, a particular solution, and it, as you already know, is calculated directly,
without any reference to that general form. We will show how one could, in principal, calculate
analytically a frequency response function for a linear model. In practice, in models of any size, it is
easier to calculate it from simulations of the model, as is suggested by the exercise at the end of the
chapter.

Many of the equations of our models take a form somewhat like the following:

(1)       y t 1 = Ay t  f t

where yt is a vector of endogenous variables, ft is a vector of exogenous variables, and A is a matrix.
If some variable, say u, appears on the right with values lagged further back than t, we can easily
introduce v defined by

(2)       v t 1 = u t

so that

(3)       v t = u t−1

and use v t in place of u t−1 while including (2) among the equations. Among the exogenous values,
a lagged variable, say x[4], can be simply regarded as the value in period t of x four periods earlier.
Thus, the presence of variables with more than one lag does not prevent us from thinking of the
model as though it were described by the equation (1).

Nonlinearities, however, such as gdp = gdpR*gdpD, cannot be described exactly in system (1).
They can, however, be represented approximately by the first terms of the Taylor series
approximation. Thus, equation (1) comes close enough to describing many models to be worth
studying.

                                                                                                     119
Equation (1) is called a system of linear, first order difference equations. The term first order refers
to the fact that at most only one lagged value of any variable appears. In the form (1), the
differences are not evident, but if we subtract y t from both sides, it becomes

(4)    y t 1 − yt =  A − I  y t  f t

and the differences on the left side are apparent. (Systems described in continuous time would have
          dy t 
the form          = Ay t   f t  , which is a system of linear differential equations. The
            dt
mathematical theories of differential and difference equations are closely parallel, except that for
difference equations there is no problem of the existence of a solution, since the equations tell us
precisely how to calculate it.)

We will now show that any one of the variables in a system of n first-order equations, such as
equation (1), follows an nth - order difference equation not involving the other variables in the
system. That is, the value for period t+n can be expressed as a linear function of the values of the
same variable in n previous periods and the exogenous variables. This fact will allow us to apply
some well-known results about the roots of polynomials to characterize the solutions of the
difference equations.

We will at first concentrate on the homogeneous part of the system, that is to say, the part of the
system obtained by dropping the exogenous variables. Let us begin by writing this homogeneous
part of the system (1) out in more explicit form:




                                                 
        y 1,t 1   a11    a 12   a 13      a 1n     y1 t
        y 2,t 1   a 21   a 22   a 23      a 2n     y2 t
(5)     y 3,t 1 = a 31   a 32   a 33      a 3n     y3 t
          ⋯                                     ⋯
        y n ,t 1  a n1   a n2   a n3      a nn     yn t

From this system, we want to derive a single equation for y t of the form

(6)    y 1, t 1 c n−1 y1, t n−1 c 0 y1t =0 .

First, we now solve the second of these equations for y 2t and substitute in all the equations, thus:




120
                                                                                   
                             0           1          0                     0
                             a a        a 22        a a                    a a




                                                                                      
          y 1,t 1      a 21− 22 11            a 23− 22 13          a 2n − 22 1n        y1 t
                               a12      a12           a12                    a 12
          y 2,t 1                                                                      y 1,t 1
                             a a        a32         a a                    a a
(7)       y 3,t 1    = a 31− 32 11            a 33− 32 13          a 3n − 32 1n         y3 t
                               a12      a12           a12                    a 12
            ⋯                                                                             ⋯
                                                                       
          y n ,t 1                                                                       yn t
                             a a        a n2        a a                    a a
                        a n1− n2 11            a n3− n2 13          a nn− n2 1n
                               a12      a12           a 12                   a 12

The matrix on the right of (7) we shall call B2. It may be obtained from the matrix A on the right of
(5) by post-multiplication of A by




                                                          
                    1          0        0           0
                   −a 11       1      −a11         −a 11
                                               
(8)          M 2 = a 12       a12      a 12         a 12
                    0          0        1           0
                                                
                    0          0        0           1

Thus, .

       B 2 = AM 2

Notice that while we have now eliminated y2 on the right of (7), it still appears on the left. To
continue the process of eliminating y2, we use the first equation of (5) advanced by one period (so
that it is an equation for y1,t+2) to write y1,t+2 as a linear combination of the equations (7). The
resulting equation for y1,t+2 does not contain any reference to y2 . If we put this equation in place of
the second equation in (7), the system will have the form




                                                      
              y 1, t1    0    1    0           0          y 1t
              y 2, t1   d 21 d 22 d 23         d 2n      y1, t1
(9)           y 3, t1 = d 31 d 32 d 33         d 3n       y3 t
                ⋯                                      ⋯
              y n , t 1 d n1 d n2 d n3         d nn       yn t

where the matrix D 2 on the right is derived by premultiplying B 2 by N2 given by




                                                                                                      121
                                          
                  1  0  0          0
                 a  a  a 13        a 11
(10)        N 2= 11 12
                  0  0  1          0
                  0  0  0          1

thus,

(11)        D 2= N 2 A M 2 .

Note that

(12)        N 2 M 2= I ;

in other words, N 2 and M 2 are inverses of one another.

In (9) we have succeeded in eliminating y2 from the system. We can now repeat this process to
eliminate, one by one, all the variables other than y1 from the system. When finished, we will have a
system of the form




                                                 
         y1, t1     0   1   0           0              y 1,t
         y 1,t 2    0   0   1           0            y 1,t 1
(13)     y 1, t3 =                               y 1,t 2
                    0   0   0           1              
         y 1,t n   −c0 −c1 −c 2        −c n−1       y 1,tn−1


A matrix of the form shown on the right of (13) is called a Frobenius matrix, and the method we
have just explained for deriving a Frobenius matrix from a given matrix A is called the Danilevsky
process. The Frobenius matrix is also called the companion matrix.

From the last equation of (13), we read, after bringing all the right-hand side to the left side of the
equation,

(14) y 1,t nc n−1 y1, tn−1c n−2 y 1,t n−2c 0 y 1 t =0


If we now try a solution of (14) of the form
             t
(15) y1t = 

where λ is just a (possibly complex) number and the superscript t denotes raising to a power, then we


122
see (after dividing both sides of the resulting equation by λ t ) that there will be such a solution if and
only if λ is a solution (or root) of the characteristic polynomial

(16)   n  c n−1 n−1  c n−2  n−2   c 0 =0

One may reasonably ask at this point: Would we have gotten a different polynomial in (16) if we had
chosen to convert (5) into a single equation involving only values of y2 instead of y1 ? The answer is
No, but to demonstrate that fact, an alternative interpretation of the transformation process is useful.
Let us return to the homogeneous part of equation (5),

(17)   y t 1= A y t

and ask, under what condition would

(18)   y t =v t

be a solution, where v is a constant vector? Direct substitution in (17) and cancellation of λ t from
both sides shows that the condition is that

(19)   v = Av .

Values of  for which (19) holds are called characteristic values of the matrix A and the
corresponding vectors are called characteristic vectors. (The half-German words eigenvalues and
eigenvectors are also in common use in English. One occasionally sees the older proper or secular
values and vectors.) Well, how does one go about finding the characteristic polynomial? You may
encounter books that tell you that for (19) to have a non-zero solution, A− I must be singular, so
its determinant must be zero, and its determinant is a polynomial in λ. That is true but useless
information, since computing the determinant as a function of λ is an enormous and complicated job
by the usual definitions of determinants when n is more than 2 or 3. Is there a simpler way?

There is, and you already know it; but to bring that knowledge into consciousness we need a simple
definition and theorem. A matrix A is said to be similar to a matrix B if there exists a nonsingular
matrix M such that A=MBM −1 . You can easily show that if A is similar to B, then B is similar to A,
so we can say simply that they are similar without bothering over which is similar to which.
Moreover, if A and B are similar and B and C are similar, then A and C are similar. The theorem we
need is simply: Similar matrices have the same characteristic roots. Proof: suppose that Bv= v
and A= MBM −1 . Then AMv = MBM −1 Mv  = MBv = M  v  =  Mv , so that λ is also
a characteristic value of A with Mv as the associated characteristic vector.

Now recall that in the Danilevsky method, N 2 and M 2 are inverses of one another, so that D 2 in (11)
is similar to A. At each further step in the Danilevsky method, similarity is maintained, so that the
Frobenius matrix in (13) is similar to A. Furthermore, λ is a characteristic root of the Frobenius
matrix if and only if it satisfies the polynomial equation (16), for in that case



                                                                                                       123
                                                
           1    0    1    0              0       1
               0    0    1              0       
(20)    2 =                               2
               0    0    0              1      
           n−1
              −c 0 −c 1 −c2            −c n−1  n−1

No two different Frobenius matrices are similar to one another, for if they were similar, they would
have the same roots and therefore have the same bottom row, and therefore be identical. If we had
wanted an equation in y2 instead of y1 , we would have begun the Danilevsky process by
interchanging the first and second rows and then the first and second columns. You can easily verify
that the matrices which accomplish these operations by pre- and post-multiplication are inverses of
one another, so that the resulting matrix is similar to the original A. Thus, the final Frobenius matrix
is similar to A and thus is the same as the one obtained without interchange of rows and
corresponding columns.

We can now bring to bear on the solution of a system of difference equations a few well-known facts
about the roots of polynomials. These are:

(a) An nth degree polynomial P(λ) – such as (16) – has n roots 1, 2, ,n , and can be factored and
    written P =−1 −2 −n  . If the roots are not all distinct, tiny changes in the
    coefficients of the polynomial will make them distinct.
(b) Some or all of the roots may be complex, that is, they may be of the form a + bi, where a and b
    are real numbers and i 2 = -1. Any complex roots occur in conjugate pairs: if a + bi is a root, so
    is a - bi.

We will consider only the case in which all the roots are distinct, since, for equations with
empirically determined coefficients, that is almost certainly the case. If the roots of A are distinct,
the corresponding characteristic vectors, V 1 , V 2 , ..., V n , are linearly independent, for suppose that,
to the contrary, one of them, say, V r+1, is a linear combination of r linearly independent ones:
                  r

(21)   V r 1 =∑ b j V j .
                 j =1
We could then multiply both sides of (21) by A to obtain
                         r
(22)   r 1 V r1 =∑  j b j V j
                        j =1


But we could also simply multiply both sides of (21) by λ r+1 and subtract from (22) to get
            r
(23)   0=∑ b j  j − r 1 V j
           j=1
But since all the  j − r 1 ≠0 , equation (23) contradicts the assumption that V 1 , ..., V r are linearly
independent.



124
                                               t       t           t
We have now found n different solutions, V 1  1 ,V 2 2 ,, V n  n , , to the homogenous system of
difference equations (5) or (17). You can also easily see that multiplying any one of these solutions
by a constant gives a solution and that the sum of several solutions is also a solution. Thus
               n
(24)     y t =∑ b j V j tj
              j=1


is also a solution. We may, in fact, call it the general solution of the homogeneous equation, for the
b j can be determined to satisfy any initial condition, that is, to give any required value of y0 , since
for t = 0, (24) becomes
               n

(25)     y0 =∑ b j V j
              j =1
which is just a non-singular system of linear equations to determine the b’s.

Finally, let x t be any solution of the non-homogeneous equation (1), which we repeat here with x as
the argument:

(26)     xt1= Ax t  f t

Note that this x t may very well not satisfy the initial conditions we will need. The general solution
of the system (1) is then
               n
(27)     y t =∑ b j V j tj  x t
              j=1
That is to say, the values of the b’s can be determined to match any initial condition. In the case of
complex roots, the roots occur in conjugate pairs and the corresponding characteristic vectors are
also complex conjugates. The b’s corresponding to conjugate vectors will also be conjugate. Thus,
all complex numbers occurring in the summation on the right of (27) occur in conjugate pairs, so that
the complex parts cancel out. Thus, though the right side of (27) may be strewn with complex
numbers, the left side, the part we actually see in the solution, has only real numbers.


       2. Frequency Response

We have given much attention in the preceding section to the general solution of the homogeneous
equations but said almost nothing about particular solutions to the full, non-homogeneous system

(1)      y t1 = Ay t  f t

or, in terms of the single, nth order equation for one of the variables,

(2)      y1, t n  c n −1 y 1, t n −1  c n −2 y1, t n − 2    c 0 y1   t   = f t 

We will now employ mainly the single equation form (2) and drop the 1 subscript, since the same


                                                                                                      125
equation right-hand side applies to any of the variables.

In the study of dynamic systems, the function f(t) is often called the input function, while y is the
output, terms which correspond closely to our exogenous and endogenous variables.

There is not a lot that can be said in general about the particular solutions, but for specific f(t)
functions, there are easily found particular solutions. For example, if f(t) is a constant, then there is a
particular solution which is constant and its value is easily calculated from (2). If f(t) is a
polynomial in t, then there is a particular solution having the form of a polynomial of the same
degree in t.

If

(3)           f t = t

where  is some possibly complex constant, then, as you can see by substitution, there is a
                            t
particular solution y t =b  where b is a constant with value

                                          1
(4)           b =     n           n−1                           .
                      c n−1           c n−2  n−2    c 0

This function is called the frequency response function for reasons we now need to explain.

As already noted, some of the λ may be complex; if so, they occur in conjugate pairs. In this
context, a complex root is most revealingly thought of in its polar representation,

      (5)      = r cos   i sin                                                                    
                                                                              i

as illustrated in Figure 7.1 to the right. The particularly
attractive aspect of the polar representation is that
multiplication of two complex numbers amounts to the                                   r
                                                                    r sin 
multiplication of their absolute values, r, and addition of
their angles, α. Thus, if
                                                                                  
            1 = r cos   i sin 
            2 = r cos   i sin                                                   r cos 
                                                                                                Figure 7.1
then

            1  2 = rs cos  cos −sin sin i sin  cos cos sin 
                   = rs cosi sin

by the formulas for the sine and cosine of the sum of two angles. With λ given by (4),



126
(6)    t = r t cos t   i sin t  .

Thus, terms of the general solution coming from real roots either grow or decline exponentially,
while terms stemming from complex roots oscillate, with the amplitude of the oscillations either
growing or declining exponentially depending on the absolute value of the root. The frequency of the
oscillation is determine by the angle α of the root; it does not change as the amplitude swells or
dampens. If all of the roots of the homogeneous part are less than 1.0 in absolute value, the system
is said to be stable.

In a stable system, a major question is how it responds to a sinusoidal input represented by (3).
According to the absolute value of b in equation (4), the frequency response curve, the system may
magnify the oscillations at some frequencies while dampening them at others.

To illustrate these ideas, we may consider a second order system with two complex conjugate roots
given by 1 = r cos   i sin  and 2 = r cos  − i sin  . For this system, the
polynomial (7.1.6) is given by

(7)     − r cos   i sin  − r cos  − i sin  =  2 −  2 r cos   r 2 =0 .

Let us take three cases, all with =/ 4 so that cos α = .70711, and with r = .99, 1.0, and 1.01,
respectively. In the first case, the system is just inside the stable zone; in the third, it is mildly
explosive; and in the second, it is exactly on the boundary. The three equations, in the form of
equation (4), are

   y t − 1.40008 y t −1  .9801 y t −2 = 0
  y t − 1.41422 y t −1  1.0000 y t −2 = 0
  y t − 1.42836 y t −1  1.0201 y t −2 = 0

These three equations were simulated from 1960.1 to 2007.4 with the initial condition that
 y1960.1 = 0, y 1960.2= 0, and y 1960.3 = 1.0 . The result is shown in the graph labeled “Deterministic
Solutions” below. All three equations show a strict periodicity of 8 periods per cycle, as could be
predicted from the fact that =/ 4 radians, so that it requires 8 periods to move completely around
the 2π radians of the full circle. The top equation, with r = .99 diminishes steadily in amplitude; the
middle one, with r = 1.00 keeps a constant amplitude; the third, with r = 1.01 gradually increases in
amplitude.




                                                                                                         127
                                                   Deterministic Solutions
                                8.9




                                -0.2




                                -9.3

                             1960         1970          1980        1990     2000
                                    a.x      a.y         a.z




(The simulations were made by running a model made from the following Master file:

      fex   y   = 0
      fex   z   = 0
      fex   x   = 0
      r x   =   1.400078*x[1] - .9801*x[2] + e1
      d
      r y   = 1.41422*y[1] - 1.00*y[2] + e1
      d
      r z   = 1.428362*z[1] -1.0201*z[2] + e1
      d
      end

and e1 was given the value of 1 in 1960.3 and otherwise 0.)

These results are, of course, exactly in line with expectations from the theory. But now let us try
something different.

Let us simulate the equations with random error terms, t , generated by the @normal() function of
G7 to be a series of independent random variables each normal (0,1). One might imagine that these
shocks would cancel out or that the result would be utterly chaotic. In fact, the result is quite
different, as shown by the graph below labeled “Stochastic Simulation”. At the beginning, the result
is quite irregular and quite similar for all three equations. Then gradually a pattern emerges, namely
exactly the 8-period cycle of the deterministic simulations. The equation with the root just outside
the unit circle is clearly exploding, but the equation with the root just inside the unit circle is not
converging to zero but to a sort of limit cycle. The equation with the root on the unit circle seems
labile, equally prone to expanding or contracting.




128
                                                 Stochastic Simulation
                              26.9




                              0.6




                             -25.6

                           1960         1970          1980       1990    2000
                                  a.x      a.y         a.z




This tendency of the system to oscillate at a specific frequency when subject to
utterly random shocks is surely both beautiful and somewhat mysterious. I will refer       /     ∣b∣
                                                                                           0.00    1.72
to it as the emergent cycle phenomenon. You will recognize it as reminiscent of the
                                                                                           0.05    1.80
cycles which appeared in running the AMI model of Chapter 1. Our analysis,                 0.10    2.07
however, can now throw some light on why it happens.                                       0.15    2.74
                                                                                           0.20    4.95
Think about what equation (3) would mean if γ were a complex number lying on the           0.25   71.07
unit circle in the complex plane. If we think of time as continuous, its real part         0.30    4.22
would follow a cosine function while its imaginary part followed a sine curve. If          0.35    1.99
time is discrete, then we observe only equally spaced points on those curves. The          0.40    1.27
particular solution, y t =b  t , looks much the same – also sine and cosine functions     0.45    0.92
                                                                                           0.50    0.71
with the same frequency as the input function – but with a different amplitude given
                                                                                           0.55    0.58
by the absolute value of b. The box to the right shows the absolute value of b
                                                                                           0.60    0.50
calculated by equation (4) for various values of γ lying on the unit circle at values of   0.65    0.43
α/π shown in the left column of the box. At a frequency of .25π, there is an               0.70    0.39
enormous, 71-fold amplification of the input signal. This frequency corresponds, of        0.75    0.36
course, to the root which lies just inside the unit circle at that frequency. Other        0.80    0.33
frequencies are much less magnified, and the high frequencies are damped. This             0.85    0.32
function is called the frequency response function of the system. It is closely related    0.90    0.30
to the transfer function commonly used in engineering analysis of systems. A graph         0.95    0.30
of the function for our equation is shown below.                                           1.00    0.30




                                                                                                    129
                                       Frequency Response Function
                         75.00
                         70.00
                         65.00
                         60.00
                         55.00
                         50.00
                         45.00
                         40.00
                         35.00
                         30.00
                         25.00
                         20.00
                         15.00
                         10.00
                          5.00
                          0.00
                             0.00 0.15 0.30 0.45 0.60 0.75 0.90 1.05 1.20 1.35 1.50 1.65 1.80 1.95
                                                         Radians



      3. Explaining the emergent cycle phenomenon

We can think of the input to the system, the series of random shocks, as the sum of T different
functions, where T is the number of periods over which the system is run:

(8)     f t  = f 1 t   f 2 t     f T t

where f j t  = 0 for j ≠ t, and is equal to the value of the shock in period t for j = t. Because of
the linearity of the system, the solution with the input f(t) is the sum of the solutions for each of the
separately. The solution for f j t  is zero for t < j; thereafter, it follows a damped sinusoidal curve
                                        t t
that can be thought of as y j t = b j r  , where rγ is the root, with γ lying on the unit circle. (In
systems of order higher than 2, there may be other terms in the solution for y j t ; we assume that all
but the largest complex root damp away quick and leave us concerned only with its contribution to
the solution.) The complex number bj is determined by the initial condition. By (8), therefore, the
output, y(t), is
                 t
(9)     y jt = ∑ b j r t − j  t = At t ,
                 j=1


where the second equation defines the complex number At. The  t on the right represents a steady
sinusoidal function with a fixed frequency. Multiplication by the complex number At in front of it
changes its amplitude and shifts its phase – that is, it adds the angle of At to that of  t – but it does
not change its frequency. At first, as the system starts up, each period brings significant changes to
At, and the output does not look much like a sine curve. As At builds up, however, as it will do if r is
close to 1, the new addition to At, which is probably typical of the terms already in it, will bring little
change to the number, so that the change in f(t) over a few periods will be determined chiefly by  t


130
and will look much like a sine curve.

An alternative way to explain the emergent cycle phenomenon is to use a different decomposition of
the input function, this time as a sum of sine and cosine functions as follows. Let T+1 be the total
length of the simulation with observations numbered t = 0, 1, 2, ... T, and, to keep matters simple, let
T be an even number and define M =T /2 . Further, define the angles

              2 j
(10)    j=        for j = 1, ..., M
              T 1

and the variables

(11)   x j t  = cos  j t and     y j t  = sin  j t for j = 1, 2, ..., M

Notice that as t goes from 0 to T, the cosine function x1 t  makes one complete cycle (except for
the last point), x 2 t makes two complete cycles (except for the last point), and so on. The same is
true for the sine functions. Now let us imagine regressing the input function, f(t) on these 2M
variables plus an intercept. Since we have T variables and T points, we will get a perfect fit and can
write
                          M
(12)    f t  = a 0    ∑ a j x j t     b j y j t 
                         j=1


(There is no problem about the linear independence of the various x j and y j variables; in fact, they
are orthogonal, so the a’s and b’s can be computed one-by-one, but these facts need not detain us
here.)

To use what we know about the frequency response function, equation (4), we just need to convert
(16) into a weighted sum of powers of a complex number on the unit circle. To do so, just define
  j = cos  j i sin  j and denote its complex conjugate by  j = cos  j −i sin  j . Then
                                                              

(13)    t                            
        j = cos  t  i sin  t and tj = cos t−i sin t

and

       a j t t          i b j t t
(14)       j   j  −       j −  j  = a j cos  j t  b j sin  j t
       2                   2

or, by defining d j = a j −i b j / 2 ,

(15)               
        d j tj  d j tj = a j cos  j t  b j sin  j t .

Equation (12) can now be written as


                                                                                                     131
                          M
(16)   f t  = a 0     ∑ d j tj      
                                      d j tj
                         j=1
as was desired.

The frequency response function of the system, the absolute value of equation (4), evaluated at each
  j will thus show how much the system amplifies or dampens the various frequencies in the input
function. Naturally, it may take a while for the strongly amplified frequency to emerge, since the
initial conditions and the terms from the homogeneous part may play a prominent role at first. But
as they fade out, we will observe the emergent cycle phenomenon.

This second way of trying to understand the emergent cycle phenomenon is connected with the
subjects of Fourier and spectral analysis of time series. The word spectrum indicates the origins of
the subject in the physics of light, while the expression white noise for a series of independent,
identically distributed random variables is a hybrid between the physics of light and the physics of
sound. In many technical studies, ranging from vibration in airplanes to patterns in the human heart
beat, it is often just as natural to think in terms of the fraction of the total variance of the series
accounted for by sinusoidal curves of different frequencies as to think in terms of the original time
series. For a given T, the share of the sinusoidal curves with angular frequency ω j in the total
variance is a 2j b 2j / 2 . If, however, we double T, there will be twice as many frequencies, and the
fraction of the variance explained by any one will go down. To get a measure associated with  j
which is potentially independent of the length of period over which it was estimated, one
conventionally concentrates on the function

                     T
(17)   s  j  =      a 2 b 2 
                    8 j       j



which is called the spectrum of the series. The real usefulness of this way of looking at a time series
is that the spectrum of the output of a linear system is the spectrum of the input multiplied by the
frequency response function of the system, as we have essentially already seen.

We have seen, I believe, that the concepts of spectral analysis can be useful in understanding why
models of the economy -- and even the economy itself -- can have a tendency to cycle. Is it useful
to look at the spectra of economic time series? I was once hopeful that it might be. And for analysis
of seasonal patterns, perhaps it is. But for the study of business fluctuations in the economy, I have
been unimpressed. All the interesting historical particularities of each cycle get mushed together.
For heart beats or airplane vibrations, the only thing that matters is the repetitious pattern. But we
live inside business cycles; we watch them develop, we follow each with its own fascinating quirks.
We have stories to tell about what caused this or that turn of a series. All that gets lost in the spectra
which, to me at least, look pretty uninteresting.

Looking at the response of a model to a cycle in an exogenous variable, however, could be much
more interesting. Most models, of course, are nonlinear, even if only in the value = price*quantity
equations. The strict linearity assumptions about the model made in this chapter will not hold in


132
practice. Nonetheless, you may find here some interesting possibilities for experimentation with
your model, as suggested in the following exercise.

                                                Exercise

7.1       Explore the sensitivity of your model to cycles in the exogenous variables. For example, if
          gR is an exogenous variable, make up this substitute for it:
      f    gRalt = gR + 100.*@sin(2*3.14159*.2*time)

This gRalt will have a sine curve of amplitude 100 with a five year period added to the historical
values of gR. Save it into a file, edit the file to introduce the variable with “update gR”, and add this
file at the ] prompt before running the model. Compare the results of the model run with this value
of government purchases with the historical simulation. Repeat the experiment with different values
of the fraction before the time variable. Does your model, or more specifically, gdpR in your model,
respond differently to different frequencies of variation in gR?




                                                                                                     133
134
                                                                                     Index


accelerator theory of investment..........................3, 35                              false exogeneity................................................74, 103
accounting model...................................................7, 9                      Fourier analysis......................................................132
AMI model.........................................................35, 73                     frequency response.................119, 125, 126, 127, 131
assigned bank...........................................................15                   Frobenius matrix....................................................122
asymptotic idiocy...................................................114                      g.cfg file...................................................................59
BasePred.................................................................115                 G7 commands...............................................................
BEA..........................................................................11                  add.......................................................................61
behavioral ratio............................................8, 16, 100                           bank.....................................................................19
Beta...........................................................................31                catch..............................................................29, 30
Build commands...........................................................                        clear.....................................................................63
    add.......................................................................34                 con.......................................................................91
    addprint...............................................................82                    do.......................................................................112
    check.......................................................16, 17, 66                       f...........................................................................15
    checkdup.............................................................82                      fdates...................................................................63
    maxit...................................................................82                   fex.................................................................16, 88
bws............................................................................67                gdates..................................................................20
chain weighting........................................................25                        gname..................................................................20
character set..............................................................60                    gr.........................................................................15
characteristic polynomial.......................................123                              gr *......................................................................60
characteristic values...............................................123                          Graph settings.....................................................64
characteristic vectors..............................................123                          lgr........................................................................66
closeness of fit..........................................................43                     lim.......................................................................29
coefficient of multiple determination................31, 43                                      mode....................................................................78
Compact inversion...................................................55                           r.....................................................................29, 32
companion matrix..................................................122                            recur..........................................................105, 112
Compare...................................................................68                     rhosave................................................................76
Compare commands.................................................69                              save...............................................................29, 31
    *...........................................................................70               sgr........................................................................66
    &.........................................................................70                 showt...................................................................47
computer prices........................................................24                        sma......................................................................96
Consumption equation.......................................2, 105                                sty........................................................................79
counter-historical simulation.....1, 16, 17, 73, 74, 109                                         tdates...................................................................79
Danilevsky process................................................122                            ti..........................................................................59
degrees of freedom.............................................31, 44                            vaxl......................................................................65
depreciation..............................................................11                     vrange..................................................................64
depvar.......................................................................78                  !...........................................................................32
difference equations...............................................119                           [] notation............................................................29
Distributed lags........................................................94                       @cum function.............................................29, 85
DoFree......................................................................31                   #...........................................................................15
Durbin-Watson.............................................31, 44, 45                         Gauss-Jordan reduction............................................42
DW...........................................................................31              Hedonic deflator.......................................................24
Elas...........................................................................31            historical simulation...................................................1
elasticity.............................................................31, 46                identity-centered modeling....................................7, 8
endogenous.................................................................2                 import equation........................................................33
exogall.reg................................................................67                inflation................................................................8, 86
exogenous...................................................................2                INFORUM................................................................iii
exogenous variable...................................................20                      inventory investment................................................32
expected inflation.....................................................87                    investment equation...........................................3, 100


                                                                                                                                                                        135
lagged dependent variable..............................109, 115                   Run commands.............................................................
least squares.................................................28, 37, 56              cta........................................................................75
Leverage...................................................................57         mul......................................................................75
MAPE.......................................................................31         ovr.......................................................................75
marginal explanatory value..........................31, 46, 55                        rho.......................................................................75
master file...........................................................14, 18          skip......................................................................75
Matrix.......................................................................48       ti..........................................................................74
Mean.........................................................................31       type......................................................................74
mean absolute percentage error.........................31, 44                         update..................................................................74
Mexval..........................................................31, 46, 55        run.gr........................................................................67
NAN.........................................................................81    run.nam.....................................................................67
National Income and Product Accounts.....................9                        run.xog......................................................................67
NIPA..............................................................viii, 9, 23     savings function.....................................................100
Non-residential structures......................................108               SEE.....................................................................31, 43
NorRes......................................................................31    SEE+1................................................................31, 46
predicted value.........................................................42        similar matrix.........................................................123
QUEST model...........................................................ix          simultaneous equation bias...............................ix, 107
Quip databank....................................................15, 25           skipall.fix..................................................................67
r-bar-square..............................................................31      SNA..........................................................................16
RBSQ.......................................................................31     Soft constraints.........................................................90
recursive regression................................................112           spectral analysis.....................................................132
Reg-coef...................................................................31     spectrum.................................................................132
regression coefficients.....................30, 31, 46, 50, 91                    Statistical discrepancy..............................................13
Residential construction.........................................116              stub file.....................................................................68
residuals....................................................................42   tautological model......................................................7
RHO.........................................................................31    transfer function.....................................................129
rho adjustment..................................31, 46, 67, 76, 78                workspace...............................................15, 57, 59, 69
RSQ..........................................................................31




136

				
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